Why aren't all modern vinyl releases 45s?

That’s a loaded question and I’m sure others can elaborate deeper than I can. But I’d say this. Not all 45s sound better than 33s, but in general if an album is being prepped for release on 45, it should mean that it is specifically mastered for vinyl,  and is a quality pressing. That is not always the case. 

To be honest, 45s are quite a pain in the butt to get up every one or two (maybe if you’re lucky, three) songs. There’s not a huge market that will invest the money and time to do that. Most record purchasers are doing it these days because they love the sound of analog, tactile feel of medium, and the artwork that accompanies the album. But the typical consumer does not have a sound system that will easily demonstrate the benefits that a high quality record can bring.

But to go back to my opening statement, there are many, many albums that sound great on 33. Michael Jackson’s Thriller first pressing. The recent Jimmy Page remasters of Led Zeppelin albums. So many more. I think the question then might become “Why do so many 33 records sound so bad?”. And this is usually due to poor pressing quality and albums that have not been mastered separately for vinyl. With vinyl delivering differently on dynamics, any album that hasn’t been properly mastered for the medium is a huge miss. 

Most releases are put out there to capitalize on consumerism and make as much of a return as possible. But there are some labels, plants, and artists that still strive for quality. 

Thanks, the poor-sounding 33s seems to be the question at hand. I have my handful of 33s with great sound stages released across many decades (1st pressing of But Not for Me comes to mind), even if that's not my system's strong suit. Mastering is the other big consideration, I have some LPs that jump off of the speakers, while you have to crank the volume on others to get them going.

Don't have the technical knowledge to further the conversation, so we wait...

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My guess is the people who sell records have figured out that there is not a big enough market for 45 rpm LPs, and that may be because the majority of us end users do not want to get up to flip LPs even more often than usual and have found that 45s are not so reliably superior to 33s as you seem to think.  Also, the producers need to charge much more for 45s, because in general it takes two discs to encode the info found on one 33, which is another reason they may not sell as well, certain pressings always excepted.

I think when it comes to reissues it has to do with the licensing agreement. When Music Matters was reissuing Blue Note jazz titles, some were done at 33 while others were at 45.