That's a good question. Why is the company that ranks 4. place in the Fortune 500 index unable to implement basic, boring but useful HTML 5 features in their desktop browser that were implemented by Opera nearly 10 years ago, while at the same time implementing them on their mobile browser?
Are they unable? Of course not. Is desktop Safari in any way a priority to them? Not really, at this point.
A long-shot, but could they be waiting for convergence of desktop and mobile in MacOS? With the upcoming Mojave, they're taking a small step in that direction. Not an excuse for these delays dating back 5+ years, but maybe an interesting question.
> could they be waiting for convergence of desktop and mobile in MacOS?
Maybe, but... honestly, that's not really good enough.
They probably care very little about desktop Safari. There are few persuasive business reasons to care about it at this point. macOS as a whole seems to be considerably deprioritized.