This could've been achieved while they were working on the animations for dual wielding. Saying they would need to do every animation all over again doesn't sound like much of an excuse for not doing it. There had better be a more sensible reason for this decision than that.
We cant give you one seeing as how they dont tell us everything
And there is always a reason for it not to be there
Bethesda makes games where they have a lot of stuff in it
They have always been a company that wanted to include everything
But they do run into problems..