Originally Posted by Nic_II

That was my first guess, too. And it does seem that current trailers use more and more music from other films (as well as popular songs) while the trailers from the 80s and 90s often had original music from someone (John Beal, John Eric Alexander etc.) who specialized in composing music for trailers and commercials.

However, that still leaves some questions, such as why the trailer is quite often missing from a new special edition of an older release that had it (to add insult to the injury, the new featurette often includes bits from the trailer with the original trailer music!), why it's often included in R2-release but missing from R1 (the marketplace for an R2-disc is, of course, smaller, but they are often released by smaller companies with less money to spare, so that evens things up a bit) and why it's often available in several other discs as a preview but not on the actual movie disc itself.

Either some studios do indeed think that the consumers don't care for it (and there are lots and lots of those who don't - but many do) and once the disc has been purchased, the trailer has done it's job and doesn't have to be included. Or there are indeed some royalties issues. A composer can give out promos of his work for free to get more work but can't sell them. Perhaps something similar is at work here? When a studio uses a trailer as a promotional item on a different disc, they'd have to pay less than if they use it as a special feature on the film disc itself?

If that's the case, as a film collector and a fan of trailers, I'd naturally hope that studios would cough up the extra dough and include the trailer. But there would at least be a good financial explanation if it's not included. After all, there probably aren't too many people like me who will get the film on R2 just because it has the trailer and the R1 doesn't - although I do know a few...