- Java has historically included things in its standard library that (presumably) everyone regrets nowadays, like CORBA support. The standard JRE is already overweight. Also keep in mind that size matters for Java - in theory, it's a client side environment that's downloaded and installed by consumers (of course, in practice nobody does that anymore).

- The standard library has to be implemented by all JVM vendors, so adding lots of stuff to it that can also easily be shipped as a library creates a lot of unnecessary effort.

- Changes to the Java standard library have to go through a committee process (JCP); again, it's probably just not worth the effort.