> does anyone know WHY Apple has let its browser go stagnant?

First, prove Apple has let it stagnate. Safari continues to improve with every release. The latest tech preview scores a 459 on the html5 test. The last released version scored a 419. It's still behind the other major browsers, but stagnating implies not improving which is clearly not the case.

It might be a better question to ask why hasn't Safari implemented whatever certain feature you want to use, i.e. service workers.