It's the perfect place to grow their audience.
We all know that the Marvel films HAVE to be a huge hit. Age of Ultron made over a billion dollars, and it still fell short of the company's expectations because a movie like that is extremely expensive. So while I might not like it, I understand why they're so risk-averse when it comes to the movies. But television doesn't have those risks! More casual fans might be interested in Marvel because they blow us away in the theaters, but we know their real strength is their storytelling. And when it comes to telling stories, television is king right now.
Why mini series?
Because it will give Marvel's writers the freedom to experiment with fewer restrictions. If they're not worried about getting a series renewed, they'll be able to take more risks. People will tune in because Marvel's name will be on the cover- but they'll stay for an interesting concept. Which means Marvel would be able to get back to what they're good at: Telling challenging stories that don't cater to the status quo.