"What exactly is the issue with java ui"
A Java UI can be written to look like anything you want it to be, there is nothing in Java that prevents a UI from looking & feeling exactly like requirements state.
But generally there are costs for Java. If used in an embedded product, you aren't using the Oracle JVM so you need to partner with a company that makes an embedded JVM. This costs money. If used on a production product, there are licensing costs to Oracle last I knew.
Besides monetary costs, there is usually a performance & memory cost to running a JVM vs something else. Not a big problem for most of the production machines (are there any left?), but for mid-range it requires more memory, a heftier CPU, and a GPU.