Image hosting used to be expensive, so forums avoided it and image hosts were scummy and filled with ads. Imgur was a first mover when cloud hosting and CDNs meant it could be relatively cheap for a small shop to do quality image hosting.

Meanwhile, reddit was a forgotten subsidiary of Condé Nast working out of a corner in Wired's SF office. Jedberg will chime in, but when imgur was founded, reddit's handful of engineers were busy keeping the site from falling over. They didn't have time to build out image hosting.