Sun faced multiple challenges, largely being a legacy product with legacy offerings in a world where price point was key, their service commanded a premium, and much existing infrastructure wasn't positioned to properly capitalize on it. That's just off the top of my head, though I'd be interested in seeing what other factors accounted for the failure.
Oracle has the kiss of death for far too much in my book. I'd consider any offering it presented with extreme reluctance.
The former certainly suffered from platform constraints and a fairly rigid 'stack', but the latter was far more flexible and various teams within Sun worked closely with several potential customers on the proof of concept. Unfortunately this fell apart thanks mainly due to the commercials and some concerns with how the platform was configured within the context of security and ensuring appropriate segregation of data.
I like that the last sentence you wrote could easily describe today; many customers still care very much about the security context and the data segregation.
Amazon hit at the right time: virtualization as a concept was being adopted by developers, storage needs were going through the roof, classic storage systems weren't scaling at reasonable costs.
Amazon's other strength is that they can use the massive revenue they have sitting around for short periods of time to build out their computational infrastructure (turning their operational revenue into capital spending). IaaS took off; SaaS somewhat, and PaaS almost not at all (except in a few well defined areas).
I can't really see what oracle woudl bring to the game. It looks to me like they are just making a play to take their existing customers and move them to their data centers.