Greenie has a valid point about race being an issue. Looking at it from a slightly different perspective, that the boycotters and others fail to recognize at times - the law of numbers might actually have hurt Kap in a way too. With someone of Kap's talent, he's capable of being a starter and probably is more talented than some starters at the moment. But having more talent doesn't mean a team actually needs you. Looking at all the teams, there's only a handful that actually needed a starter:
Giants: Nope, Eli Manning
Cowboys: Nope, Dak
Eagles: Nope, Wentz
Redskins: Nope, Kirk Cousins
Bears: Yes/No. Could have signed Kap in March, but instead signed Mike Glennon to a large contract before the draft and drafted Trubinsky
Lions: Nope, Stafford
Packers: Nope, Rodgers
Vikings: Nope, Bradford/Bridgewater
Falcons: Nope, Matt Ryan
Panthers: Nope, Cam
Saints: Nope, Brees
Bucs: Nope, Winston
Cardinals: Maybe, but Carson Palmer's still around
Rams: Nope, Jared Goff was just drafted last year
Seahawks: Nope, Russell Wilson:
Jets: YES, even though Petty/Hackenberg are still w/ team
Dolphins: At FA, Tannehill was still w/ team. After injury, signed Cutler due to familiarity with offense. Also didn't sign Kap b/c of Castro comments
Patriots: Nope, Brady
Colts: Nope, Luck
Texans: Nope, Watson
Jaguars: Nope, Bortles (though he ain't any good)
Titans: Nope, Mariota
Ravens: I guess, but Flacco's still with the team
Steelers: Nope, Big Ben
Bengals: Nope, Dalton
Browns: Yes (even though they're starting the rook for now)
Chiefs: Nope, Alex Smith
Broncos: They have Siemian and Paxton Lynch and Osweiler now (tho Kap's arguably better than those 3)
Raiders: Nope, Carr
Chargers: Nope, Rivers
TL;DR, there's only like 4 or 5 teams out of the 32 in the league that actually could have made use of the QB. So when reporters and people on twitter claim everyone's refusing to sign him, it's not necessarily because of want, but of need.
Now, the easy question is what about being a backup? What the activists and others fail to realize that the backup QB role is not always about talent. Take away the Vick/McNabb tandem in Philadelphia, but you almost never see an uber talented backup QB. The role of the backup QB has one of two purposes: either be a vet and be a mentor to the starter or be an up-and-coming rook and learn from the starter.
Additionally, your backup QB tends to be behind the scenes and not newsworthy of any note. It's the case in almost any sport. Likewise, the starting QB has to be the leader of the offense and rile up the troops.
Based on the above, you see that Kap doesn't really match the true job requirements of a backup QB. He's one of those players who hasn't had the vast experience of a vet but isn't a spring chicken by any means. Likewise, he's just going to garner a lot of attention as a backup QB that likely won't be any of his fault. His supporters might actually be a hindrance in that aspect. You just know that any team that has him on the roster will be answering a lot of non-football questions that are naturally going to be a distraction to the team. It just simply is. It'll be the same with the Melo trade rumors all season. If you're a rebuilding team, you don't want that. A playoff team, nope.
Now of the 5 teams that could actually use him, we can take the 49ers out of the running obviously. Again, I don't think Miami was ever realistic. I think the Jets could have been an interesting situation, but the owner doesn't want any part of him (it's been publicized). Don't really get why.
I do actually think the Ravens were a true candidate, but his GF pretty much ruined that opportunity for him (when I say that his supporters can be a hindrance, just look at that example). I think the only real way a team can take a chance on Kap is to have an influential NFL ambassador (think Tony Dungy) or team ambassador (HOFs, etc.) personally talk to the owners and vouch for him.
However, the issue of kneeling for the flag does not have unanimous support within the African American NFL Community and any methods of repairing that relationship has been destroyed due to Kap's supporters publicly rebuking them at any given opportunity. While Colin has done a lot within the community of donating to various causes and educating the youth, he should probably start doing interviews to allow him an opportunity to speak for himself rather than having other people speak for him.
Until the above happens, it's not likely he'll play another game in the NFL anytime soon. Sadly, race is a part of it, but it's not the only determinant.
BAF- Houston Rockets:
PG: DeAaron Fox/Jeremy Lin
SG: Waiters Island/Wesley Matthews
SF: Jabari Parker/Wilson Chandler
PF: Julius Randle
C: Nikola Vucevic/Joakim Noah