Finding a reason for Claire to be in Tall Oaks wouldn't even be that hard (for all we know it might be where she lives, or maybe she was going to meet Leon there), but how would the story have worked? Leon is just a side character in his own campaign, it's really all about Helena and her beef with Simmons, so with Claire in her place, Simmons would have to use Chris as leverage, which in turn makes you wonder how exactly Chris's own campaign would have worked out, not to mention that Claire would be useless in Simmons's ploy to assassinate the president unless TerraSave agents somehow have the required security clearance.
So... while I'm usually all for reusing old characters instead of introducing new ones never to be seen again after their one and only appearance, in this case, if they really wanted to tell this story, it was justified for once. I guess Jill quitting the BSAA after the nasty business with Wesker and becoming part of the president's security detail would be somewhat believable by RE standards, so she could have been in Helena's place, but then fans would have rioted over having her and Chris in the same game, but not as partners. That way at least, the scene from the fake Revelations trailer where they're pointing their guns at each other could have become a reality, with their teams fighting over Ada / Carla.
I have to say, though, it would be awesome if RE6's four teams consisted of Leon and Claire, Chris and Jill, Sherry and Wesker's son, and Ada. All the main characters (or in one case, their offspring) reunited in the same game... One can dream, right?