This is what they said,quote from IG's community manager:
''Long story short: Sony came to us and asked if we wanted to work with Marvel. Sony and Marvel let us decide what character/universe we wanted to make a game based on. We chose Spider-Man.
Marvel went to Sony and they went to Insomniac during 2014. Marvel wanted Sony to make a game to be treated like UC/GoW and Sony chose IG for the job. IG were given the chance to choose which character they wanted, they chose Spider-Man as he was everyone's favourite. It was always an exclusive.
https://www.psu.com/news/insomniac-chose-spiderman-ps4-marvel-character/
A quote from Sony:
''Marvel comes to third party, says hey Drake and Adam can we do something here. And what they said is we would love to see a game treated, one of our properties treated, with the reverence that first party treats their heroes.
Avengers is 100% multiplat as it's a completely different deal.