[Nick Zwar]

Probably writing for the ballet is not so popular among film composers because it is on the one hand too similar to film work to really count as a proof of variety or helping to be regarded as serious composer (maybe one of the reasons why film composers write for the concert hall?) and too far off to feel really at home technically? Just a guess.

I think there is something to it. Also, let’s not forget that classic Hollywood film composers are among the highest paid composers there are. So they probably get a lot more money from doing movie jobs than they could get from a ballet commission. So if they do it, it would have to be a passion project. But if they did a “passion project”, why not do something different from film work? Write something more formal, like a concerto or a symphony. So as you said, in some ways, ballet may be too “similar” to what they do on a day-by-day basis.
Also, depending on what you composer, a ballet is more difficult to perform than a symphony, as you don’t just need the music, you also need a choreography. And then an orchestra and dancers, so it adds up.