Portal 2 (Spoilers Possible)

[Vipershark]

That's because it essentially WAS a tech demo.
the original portal was originally intended as nothing more than an additional game in the orange box. it was never really expected to become the blockbuster that it did.

It had a relatively tiny development team working on it; now that it's established itself as a blockbuster IP, it became a, uh... "full scale" project, I guess.

In other words, this is the blockbuster-level game that the original portal would have been had it not been an extra for the orange box. ...Except, you know, a sequel.