Because AMD only every had one strategy for software:

Look at it as, we're David against the Goliath, we're going to utilize open-source because that's how you win.

Then make zero effecting trying to increase adoption, which then result in reducing support for it and eventually giving up.

Because GPUs require a lot on the software side, and AMD sucks at software. They are a CPU company that bought a GPU company. ATI should have been left alone.