I doubt WB would legally be able to do a MGM 90th, while there is a MGM company around (who may be doing their own).
I'm pretty sure that is a primary reason. I've noticed that WB has never released an MGM collection that advertised itself as such on the front packaging. Usually, they are called "Classic Hollywood" or "Dream Factory." Especially since the current MGM company still makes a few "MGM" collections of its own, even though they are almost always U/A films, like the "MGM Classic Musical" set, which is 100% U/A. However, I agree in this case that the sets should have had a stronger WB slant because they've released enough of actual WB films to easily fit into a 100 title WB set. I'm not sure about blu-ray titles though. I guess they'd have about 50 appropriate titles on hand including new releases, but much of their "Golden Era" is still largely unrepresented on blu-ray. But really, I don't think its a real issue considering the people who will likely be getting this set, which is probably designed for entire families with different tastes. I doubt hardly anyone plunking down the cash for these sets are doing it out of Warner Brothers purism, but because there are a lot of movies he/she thinks would be a good buy or are interested in.
As for The Philadelphia Story, which is MGM and not WB, it is included in this up coming collection along with Bringing Up Baby.
That just seems to be part of the DVD collection.