I don't know why they can't just leave routing the tremolo spring cavity until later so you can still specify it as a hardtail if you want. They do other routing later, eg pickup cavities, so why not the bridge too?
The do make hardtail Strat bodies, but it's true that they make far more with tremolo cavities.
I suppose the "problem" is that if you wanted a trem you'd end up with a large, unpainted hole in the back of the guitar, which many people wouldn't like - everyone uses a scratchplate, but it's quite common not to bother with a backplate.
And I think the bottom line is, most people prefer a Strat with a trem (although I'm a hardtail man myself). I'm sure they base their production on what actually sells.