Why did alhazen invent the camera obscurer?

Because it was easier than setting up a tent.

In ancient Egypt, people would put up tents in the middle of the day to shield themselves from the hot sun. Sometimes tents get holes in them, and the light streaming through the hole would sometimes focus itself on the other wall in the tent. People thought the images thus created were neat to look at, so someone eventually invented a permanent "little tent" - the camera obscura.