If I recall correctly, which I often don't lol, that was the same in SC1. My guess is they kept it like that to please the purists.
Maybe there will be an option to increase it in certain MP matches.
I suppose it means you have to try and tech up fast to get better units and also upgrade things quickly. That or rush with loads of weaker units.
I just had my best game on kulas ravine, i was protoss against zerg and terran. chinese x5 AI. I managed to expand but then got my initial base wiped out my a zerg army, i rebuilt and expanded again, this time using blocks of ion cannon things to defend. I built 15 or so upgraded void rays and in came a battlecruiser wave with some grouond units. Ion cannons were pretty effective against them with the void rays but still nailed me. - How do you upgrade the Ion Cannons? It seems like the only Protoss ground defence really
A mass of hydralisk then came in to finish me off. This is by far the best game out at the moment, so many different stragegies