My guess is that they only had a small team working on it, since their main focus was Tekken and, at the same time, they couldn’t really work out how the rest of the SF cast would be integrated. Shotos/motion characters were probably fine, Akuma in T7 is likely the result of that, but they probably were stumped with charge characters.
To be fair, I think it’s a good idea to delay TxSF’s development for now, Namco should be spending all its resources to release T7 as soon as possible, and having some of the Tekken team devoted to a whole another project, let alone that they were basically developing two Tekken games at once. If T7 is indeed the last Tekken game (or at least the end of the Mishima Bloodline saga) - it makes sense to let the series rest for some time and then pump out TxSF, since the SF5 hype will also stabilize by that time (assuming TxSF is coming out ~2019, depending on how finished the project is today)