The Marvel contract must be too costly.
No, it's reportedly because of the competition from the Disney+ streaming service. Having MCU shows on Netflix when there are other MCU shows and movies on Disney+ would essentially be advertising for a direct competitor, and the profit (i.e. increased number of subscribers) they get from the Marvel shows isn't high enough to offset that loss. So this is a domino effect from Disney starting its own proprietary streaming service.
This has happened before -- corporate maneuvers killing good Marvel shows. The animated series
The Spectacular Spider-Man was cancelled after two seasons because of a contract deal where Marvel got control of the Spider-Man TV rights in exchange for Sony keeping the movie rights, or something like that, and Marvel ended SSM (which was from an outside production company) so they could make their own in-house Spidey cartoon, the vastly inferior
Ultimate Spider-Man. (Although that's recently given way to a new show called just
Marvel's Spider-Man that's much better -- and ironically has Spidey's arch-foe Norman Osborn played by the same actor who was Spidey on SSM.)
I suspect the number of viewers is very high on the Marvel shows (they are always on the trending/popular lists), but that's not how Netflix makes its money.
Right. It's a subscription service, so the key issue is how many subscribers come
just for the Marvel shows. Their research found that most of the viewers for those shows were already Netflix subscribers, so the presence or absence of those shows on Netflix wouldn't make a massive difference to their subscription numbers.