There are couple of reasons for that.
1. php itself is a scripting language. Nothing more nothing less. If you want to get more "enterprise features", then you'll have to rely on 3rd party frameworks and APIs.
2. php struggles when it comes to scaling of applications unlike java or .net
3. php has less libraries support.
4. php enforces bad coding practices.
5. Less tool support.
With that being said, it still is a wonderful language to have under your belt.
