If that poll were done somewhere else like Comic Vine, Comic Book Resource, or perhaps even here, I guarantee the results would have been a lot different and Venom, Carnage, and Gambit probably wouldn't even be in the top 20.
when it comes to the Marvel side it seems like they're try to promote the majority of their characters and put in more characters that are actually revelant. I mean Ghost Rider has a sequel. Hawkeye's gonna be in Avengers. A Doctor Strange movie has been talked about, but hasn't been officially greenlit. I'm assumming Rocket Raccoon was Capcom wanting another unusual Marvel character.
Let's take a look at some of the Marvel vets
Cyclops - Is still pretty revelant, but is currently despised by several old school X-Men fans and he only can do one thing in the game and shoot beams which is kinda stale due to how writers have being writing him.
Gambit - Not that revelant. I like him, but let's be honest here, the only reason he became so popular was because of the 90s X-men cartoon. People can't even write for him except Liu. X-23 is the only place to read a good Gambit.
Venom - Fans want Eddie Brock, but people who don't read comics don't even know he hasn't been Venom for years. And Marvel probably wants to push the Flash Thompson version. If that version got in people would have complained. So Marvel ignored him all together.
Rogue - The version most people are familar with is the 90s version and she no longer has Ms Marvel's flight and super strength. She's more like the movie version.
Usually it's the people who don't read comics being the ones who only want characters they know and won't give lesser known characters a chance. By their mindset Strider, CapCom, Jin, Blackheart, and Thanos never would have been in these games.