Why HiFi manufacturers don't make active crossovers anymore?

The two most important considerations for a stereo are your speakers and the room that they are in. Passive set ups don’t allow you to dial in the speaker room interaction like active crossovers do. That’s the real advantage in my opinion. I use REW with a high end microphone to analyze the room. I’m able to add 12 biquad filters to each channel with my mini DSP. The distortion tamed by this process is unbelievable and the sound improvements truly amazing. Most consumers don’t want to mess with the process but it only requires probably a day of messing around and then it’s set which people argue is a Sisyphean task and never ending. You need to add multiple amps as people have noted and have multiple subs to really make an active set up effective as most distortion and negative room interactions happen at lower octaves. 

I've used both fully active biamped (Snell Type A II Imp), passive biamped (Martin Logan Prodigy, with level adjustment for woofers), and regular passive systems.  In reality, every system with a subwoofer is a passive biamped system.
In general, if you have the ability to do the setup, I think a full active system will be superior. I knew a guy in the 80s with a full Levinson HQD setup: 3 way active biamp setup of Hartley woofers, stacked quads, Decca ribbon tweeters, 6 ML2 amps, 2 LNC2 xovers, 2 ML6 mono preamps. He was very capable and the sound was excellent.
That said, I will always observe the speaker maker's recommendations for slopes and use an RTA with pink noise for initial setup, tweaking afterwards for taste.
One aesthetic issue is the quality of the crossover.  Would you want to put in line, between the exotic amps and preamp, a $200 cheapo xover? Probably not.
The new DSP based xover systems, assuming excellent DACs, would be a great solution. You'll need a speaker system with access to the driver terminals and the ability to eliminate the passive elements from the signal path. The DEQx, while limited to PCM at 96K output, has SPDIF digital outs for each of the 3 xover output bands.  This means you can create as elaborate a digital system as you like: Streamer->DEQx->3 MSB select->6 MSB mono amps. Wow!

hifidream what MiniDSP are you using? I am thinking about making some two way systems and providing some sort of preprogramed DSP with them. At $200 the MiniDSP 2x4 price is certainly right for doing this. What is the quality of the output?

I have used active electronic crossovers for the last 30 years on my Magneplanars, first on my MG2's and later on my 3's, and now on my MG20.1's. I used the Bryston 10b and 10b sub to drive my system in the triamped mode for the last several years on my MG20 + subwoofers system. Two years ago I replaced my Bryston 10b's with 2 of the Pass Labs XVR1 crossovers to run my MG20,s and subwoofers. Today after a year of trial and error listening I have settled on settings that are very satisfying. I ended up using crossover points and slopes that were close to, but not the same as the settings Magneplanar recommended as a starting point. The Pass Labs crossovers are very complex, but they also allow extremely accurate settings for the crossovers. The Pass XVR1s can set the crossover frequencies, slopes, high pass and low pass frequencies, Q factor at the crossover, and also allow independent settings for volume on all crossover sections. Pass Labs created these crossovers to use in testing their equipment during the engineering process.

I knew that I could use the manufacturers crossover setting recommendations to begin with so a lot of trial and error was eliminated this way. Some things I learned along the way, at least on my system: 6db/octave crossover slopes sound best. The Q factor settings did not affect the 6db/octave crossover slopes, per the included manual for the crossovers. The sound was better (in my system) with the MG20's and subwoofers running in parallel, not in series. i.e. the bass panel of the MG20's is not cut off on it's low frequencies, but allowed to run down to its natural cutoff. The subs and main speakers sounded best with the highest slope setting in the sub crossovers, 24 db/octave. Low frequency cutoff on the subwoofers was 48Hz, which eliminated a room mode at 60hz in my system. I found that using low frequency settings on the subwoofers sounded best with steep slopes.
I think that Pass Labs website has an electronic copy of their XVR1 crossover manual. This does a very good job of explaining the multitude of settings available on the XVR1 crossovers. I Highly recommend downloading and reading this for a reference manual for anyone contemplating Bi or Tri amping their speakers. The Pass Labs XVR1 crossovers have over 1 million crosssover setting combinations, so the process is daunting, especially if you don't have a starting point in the beginning.
As a suggestion: Use the lowest frequency you can for your subwoofer low pass setting that allows the subs to "fill in" the main speakers low end response. The subs will sound best with steep cutoffs like 24db/octave. 6db/octave slopes sound best on the main speakers, and give the best phase response for the speaker's cutoff sections. Calculate your room resonance modes (based on the LxWxH dimensions) and avoid sub settings that will excite these modes. The room resonance modes are the cause of "boominess" in your sound room. For example, my room is 18 feet wide and the room mode for the width was approx. 60Hz. I first used a sub crossover frequency of 60hz, and struggled to reduce the "boom in the room" until I lowered the sub crossover to 48hz. This lowered the sub output enough at 60 Hz to eliminate their contribution to that mode. Then I was able to move the main speakers around to minimize their contribution to the boom.
FYI - I still own the Bryston 10b and 10b sub crossovers and would like to sell them to another person wanting to biamp or triamp their system. I don't use them now, and it is a shame to leave them unused on the shelf. These crossovers analog, not digital, and are excellent sounding, and easier to set up than the Pass Labs XVR1's are. The Pass XVR1 do sound better, though.
This post is long, but I hope helpful to you in the use of electronic crossovers in your system.

I agree whole heartedly that you can acquire a much better synergy of components by utilizing an active crossover!  Being a DIYer myself, my system is composed almost entirely of diy components and it sounds absolutely fabulous!  

My active crossover is a Sublime audio K231.  It is completely customizable and they are a great company to deal with.  Don't let the $500 price tag fool you, this is a top quality component!  

Would be happy to answer any questions in a PM, but don't want to get into the back and forth on this website!