The KeePass model of development is not very appealing to contributors or anyone who might want to review the code for security. KeePass has no official public repository and only has a dump of the source code for each version the author releases. (Although some people have turned them into repos, the lack of proper commit logs means it's not very easy to see exactly what is being changed[1])
On the other hand, KeePassX is developed in the open like most open source software. I think I'd rather trust that people are reviewing the code here.
[1]:https://github.com/dlech/keepass2/commit/6c87c0a9c8e34558de9...
It's "cross compatible" only in as much as the .NET Framework is, i.e. not much. Mono is one big hack to bring what is a Windows-based framework to other platforms.
KeepassX is built on Qt, which is a truly cross-platform toolkit.