Is this a deliberate strategic decision (i.e. to drive Apple users toward native apps?). Or pure Hanlon's razor?
Apple, Microsoft, and Google all have different approaches to the consumer software that they put out. Microsoft values backwards-compatibility above all else, and Google values the bleeding edge and experimentation. Apple values predictability and ease of use. For example, Google created and deployed SPDY because they thought they had a better way to do HTTP. Eventually this turned into HTTP/2, but in the meantime Google is now faced with supporting both SPDY and HTTP/2, or deprecating and removing SPDY from Chrome. Apple, on the other hand, waited for the HTTP/2 standard to be debated and ratified before implementing and distributing it to end users.
Safari is released infrequently, but when it is it only contains (mostly) solid features. However, if you value the bleeding edge and are willing to tolerate some amount of breakage, you can always install the Safari Technology Preview, which releases much more frequently.
Safari is like when MS let IE sit at v6 for ages while Mozilla et al ran ahead.
Chrome is IE from back around the first browser war between MS and Netscape, by introducing new stuff before it has been properly ratified.
That said i have the impression that the stewardship of Webkit was what forced Google's hand in all this, by taking forever to accept patches etc.
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.
478 on Firefox 55.0.3
and here's the kicker, tested on an ancient pc from 2004 with Pentium 4
That said this is the weakest score of the top 4 browsers:
519 Chrome 57
518 Opera 45
474 Firefox 53
473 Edge 15
Mozilla is generally a safe option.