The problem is probably hidden in the "simply" part. A huge IT infrastructure built on one tech stack isn't easy to migrate to another one, it'd cost a lot and all those things that currently "just work" might stop working. And then you're on a new tech stack and who knows whether it'll work as well as Oracle has been so far? As long as Oracle keeps the lights on their existing customers will likely stick with them out of inertia.

That won't necessarily get them new customers, though, and if those new customers instead pick some other tech stack they now face the same challenge trying to get them to come to Oracle. So we'll see how things work out for them in the long run.