Why aren't all modern vinyl releases 45s?

Nah, 45 is barely any better than 33 1/3. Just wait a little bit. I'm working on producing high fidelity 78 RPM discs. Two and a half minutes or pure sonic bliss. They will sound so great that the rest of your system won't matter. Coming soon

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It’s not worth issuing on 45 rpm vinyl unless it’s a top notch recording mix. I have a few from MoFi and Analogue Productions and they are superb. Very quiet, accurate and tons of image. Dream with Dean (Martin) by Analogue is amazing. But I don’t buy many 45’s due to the cost and as Blisshifi says having to turn the record over after just a few songs. 

Why aren't all modern cars replicas of the Mazda Miata?

A valid question, though I've learnt to save what remains of my hearing with earplugs when the roof is down. I bought an NA in 1991 and have put nearly 50k km on her during the 31 short summers since.

As for 45rpm vinyl, buy one of Reference Recordings releases of Doug MacLeod and you'll quickly see why it would be wonderful if everything sounded like that!

Reference Recordings are not comparable to any average 45. First, they’re not reissues; they are derived directly from live performances, but I agree they’re marvelous.

I prefer 33 RPM reissues because I think 45s are a PITA in terms of enjoying music. I’d rather sacrifice a little bit of extra quality (and I question that’s always true anyway) if I don’t have to get up every 5-8 mins. I have a ton of 45 RPM reissues, and for not-so-small handful of them I also have the 33 RPM reissues that have come out since from the same place (e.g. MMJ and AP). Guess which ones I play? Yes, the 33s. I want to to play my system to listen to music, not the other way around.

I also listen to classical a lot and refuse to buy them as 45 RPM reissues. I bought one and it never gets played.