Why are rights to movies/TV series split up to different distributors?

NEON GENESIS EVANGELION  - ADV
EVANGELION Movie - Manga

RUROUNI KENSHIN - Media Blasters
SAMURAI X - ADV

BLACK JACK OAV series - Central Park Media
BLACK JACK MOVIE - Manga


I'm sure there are several other examples (e.g. COWBOY BEBOP movie and
TV series), but why is the series handled by one distributor and the
movies handled by another? When the rights are sold, doesn't it make
more sense to buy all the rights at once rather than just the TV
series or movie?