There are probably any number of reasons, valid or not. The easiest to cite is "Who do I call if we have a problem with OpenJDK?"

Outside of OpenJDK there are other issues that make this a hard push for Enterprise customers. I haven't seen a large installation that uses memcached. Most go with something that is tailored for Java like Terracotta's bigmemory. Again it is "Who do we call if it breaks?"

Heroku is in an interesting place here I think. They are certainly on the right trail but this is VMWare's back yard now. Being a polyglot platform is probably a disadvantage for Heroku in this arena. If you look at VMWare they have a complete system tailored to work with Java. The hardest hurdle I see is that VMWare's solution can go from developer size deployments to production all on the hardware owned by the Enterprise. Heroku can't do that and I assume will never do that. Not "owning" the entire system will be an uphill battle for a lot of Enterprise customers.