Comcast is a tremendous hydra, with countless subsidiary heads to manage and pay for all at once. I don't believe the same heads that designed Universal Kids were the same ones to create or slash the budget, set the deadlines, etc. It is a failure of the broader company to not account for these things, and I agree that the park should've been designed around the budget they actually were given - but again, I doubt it was nearly that simple.
This doesn't feel to me like an Evermore Park-style situation, where UDX just went for it and starting building stuff until they were out of money. It feels like they had a very specific plan and budget going in, that got trimmed over time as the heads holding the pocketbooks saw the economic developments of the last few years and hedged their bets.
The people designing these parks want to make a good product, and the initial ideas likely did fit within a modest budget. Had they cut one of the lands from the start and put that budget towards ensuring the other lands were guaranteed to look great, that same budget might've just ended up cut anyways and we'd be in the same position as we are now. I agree that it was short-sighted of them and the park is worse for it, but that's how these things go sometimes.
That's my current hypothesis, at least.