Can I ask, what is with all the love for Miles Morales? To my mind, and I would argue the majority of the audience out there, Peter Parker IS Spider-Man. Anything else - at least to me - is a substitute. I might add that I bailed on the ultimate line not long after they introduced Miles, it wasn't Spider-Man to me. Let's face it there's only one Superman (Clark Kent) or Batman (Bruce Wayne), even if there have been pretenders who've taken their place on occasion. At the end of the day if the Miles Morales Spidey appears then I'm going to feel short-changed and given Sony's attitude to the Miles Morales character I really don't know why Marvel had to jump through so many hoops to get the rights to use him and if Marvel use Morales in the MCU we have to forego a third Andrew Garfield ASM film made by Sony. As for Peter Parker's ethnicity why can't he be African-American or Asian-American, just because their are a lot of other ethnic minority characters running around the MCU, what's another one?
Miles is just different. Think about it. Tobey Macguire last Spider-Man movie was in 2007. Andrew Garfield's last was 2014 and now we are getting a new Spider-Man for the films in 2017. People want a change and Marvel's turned everything they touch to gold so why not? Personally I'd like to see Ben Reily/Scarlet Spider on screen but that's not happening. Miles has a fanbase and he could use the boost to his profile. Before Superior Spider-Man replaced the Amazing Spider-Man line back in 2013. Miles on Ultimate Spidey was outselling Amazing with Peter.
I don't know, outside of the comics community I'm just not seeing much of a fan base for him. I certainly doubt Marvel would have found it as difficult to secure the rights if they planned on using Miles Morales. If you look at live action comics adaptions be they TV or films the powers that be tend to go with the established popular version of a character. For instance, flashbacks and alternate reality or what-if stories aside, there hasn't been a comic featuring Dick Grayson as Robin in over thirty years, not in the main stream continuity. There have been two versions of Jason Todd (pre and post crisis), Tim Drake, Stephanie Brown and now Damian Wayne. There was also the female Robin from Frank Miller's Dark Knight graphic novels and I dare say someone will chime in with someone else who I've left out. But when it came time to insert Robin into the films, instead of going with the then current side-kick Tim Drake, they went with Dick Grayson , albeit one who allegedly contained elements of the others.
As for Miles Morales comics out performing Peter Parker ones, I'm just guessing here, but it could all be relative, i.e. the Peter Parker ones were going through a bad patch and simply performing even more badly than the Miles Morales ones?