One problem with cartoons made for movie theatres is that they were made in random lengths. In a movie theatre, it doesn't matter a hell of a lot if the cartoon runs 6 minutes and 43 seconds or 7 minutes and 9 seconds or whatever. On TV it does matter. That was one area (besides doing local commercials) that kiddie show hosts in the old days covered; they wrapped up the loose ends and made it come out even. (Back then, kid shows might include not only cartoons, but cliffhanger chapters, comedy shorts like Laurel & Hardy or the Little Rascals, etc; again, all made for theatres.) Without a host, cartoons have to be clipped, sped up, or otherwise scrunched to fit.
Another factor working against old cartoons on TV is the prevalence of cartoons on DVD; major-studio box sets of Disney, Warner, MGM, Walter Lantz, and UPA cartoons, all state of the art quality transfers; and the innumerable el cheapo public-domain discs, quality all over the map but bought as babysitters.