Here's why some films are "ignored"
There are several complaints about certain movies being "left out" or "forgotten" but if you know the hurdles involved (aside from just budget and time) then it's obvious why some notable slashers are MIA.
The film clips have to be licensed from the rights holder (at a cost of $X/per second). This alone can be complicated for many films (particularly regional and DTV movies) because the rights have changed hands so many times that it's unknown who currently owns them and the paper trails are too complicated for anyone other than the courts (too many buy-outs and bankruptcies during the VHS market crash). Any slasher film buried in red tape cannot be featured since 'fair use' doesn't apply to a commercial project like this. On the other hand, more mainstream slashers like F13 and Halloween are extremely expensive to license and can eat up a huge portion of the budget.
Depending on the contracts of those involved, you may also need the permission of the actors that appear in the clips you use as well as the composer of the music featured in the scene. Many will charge a fee for this. Tina is missing from all the NOES footage used because Amanda Wyss was asking for too much money. Tom Hanks on the other hand actually gave permission to use his scenes from "He Knows You're Alone" for FREE, but MGM's licensing fees were too expensive. If it weren't for the director's personal photos that whole segment would never have existed.
If an actor is a total *beep* and flat-out refuses permission (Laurence Fishburne) or simply can't be reached (Johnny Depp) then you can't use any footage involving them. This is why many are missing from the "before they were stars" segment.
Then you have films where the licensing is easily obtained and affordable, but those involved either can't be found or simply decline to be interviewed about it, leaving nothing to build a segment around ("Student Bodies", "Pieces", etc.).
The only movie that's missing for reasons other than the above is "Black Christmas" which had a segment that ended up cut. Personally, I think the "modern" segment could've been trimmed down significantly instead if they really needed to lose a few minutes but that's my only real complaint.
Obscure horror movies:
https://www.youtube.com/user/TheBurialGround4/videos