Yeah, going full kerolox in the first stage would have been better. I think it was because of politics, in particular pressure from the French. They were (and still are) leading the project. They wanted solid fuel to subsidize their military rocket developments, and kerolox would have made that unnecessary. This was the constraints I was talking about. In aerospace often times politics are more important than the engineering itself.

In A6 you can see this too. At the beginning it was envisioned as having a purely solid first stage, but the Germans wanted to keep building large tanks. Basically it was planned as a larger Vega, but instead we've got a shrunken A5.