It wasn't really a full game, you understand. It was a mixture of a couple of things:
A: it was a bunch of students from Digipen who had thought of something new, that Valve had liked the concept of, so they hired them. It was their first professionally published game, it's probable they weren't expected to make a game that rivals Valve's other achievements!
B: it was more of a Showcase of idea's, than a game. They had this brilliant concept with the portals and wanted it out there in the community. It's almost as if all they've done is given you a bunch of tools to make your own levels, then a bunch of idea's for what you can do (levels 1 - 19) and said "go for it guys, go crazy". Valve are so very community orientated it's crazy.
So it was short, but it was exactly what they wanted/needed.