Banks are conditional and have 30 days. Cash offers have no conditions, literally sign, wire money, and hand over the keys. Conventional loans, backed by HUD, need to go through a process. Because ultimately it's HUD who owns these loans and they are, well, slow as any government bureaucracy.
Before these times, cash offers came with a slight 5-15% discount because of the convenience and speed. Now, cash offers are overbidding other cash offers who are overbidding conventional loans. This happens in the hottest markets; South Florida for example.