It's because Portal was a test bed.
And it wasn't stupid, just short. That's probably the reason it wasn't co-op, because 1. It was a test and 2. It was to short. Hope I helped.
Excuse me while I facepalm.
The first reason, as stated above, was because it was a test bed. When Portal was first drafted, Valve didn't know how well it would be received by the general populace, and stuck it in the Orange Box to be supported by Episode 2 and TF2. When Valve realized that the game was popular, they began drafting ideas for more Portal content, which included stuff like a map pack or other DLC. They actually tried another gameplay mechanic called FSTOP (we don't know much about it), but when early playtesters complained about the fact that a Portal game had no portals in it, they added it back, with nearly the exact same gameplay mechanics as before. The only thing they removed were the High Energy Pellets, which was replaced by the Thermal Discouragement Beam, and they added the Mobility Gels, the cooperative mode (one of the ideas drafted during Portal 2's conception) and (fairly annoying stuff, IMO) portal autocentering, a new crosshair that tells which portals have been placed (as opposed to whether a portal can be placed on a surface or not), and outlines designating where portals are. Otherwise, the gameplay has not changed at all. It's short, not stupid.