This has been on my mind for months, but I just didn't have an angle until now. Rainbow Dash wants to join the Wonderbolts and the Wonderbolts need to be compared directly to Celestia's armored pegasi as a military unit.
If the Wonderbolts are the premiere flight team in Equestria, then they are more than capable of handling themselves in an aerial fight. Considering how scrappy Rainbow Dash is, she's the perfect choice. But didn't she say that she didn't finish flight school in the show? I thought she did. This might be a real reason that she hasn't made it into the team yet.