There are a lot of infrastructure and planning reasons why, in my opinion, it was deferred. Again, Apple is rarely on the cutting edege of technology, whether that be first to adopt 4G LTE.
Market research shows only a sliver of homes had 4K TV's by the end of 2016, which was a full year after the ATV4 launched. Apple is a conusmer based company now, and with the ATV already having a small market share, I'm sure the cost/benefit analysis to bringing 4K to the market in late 2015 wasn't worth it.
Additionally, 4K uses H.265 for streaming, which is basically the only way to get content onto your Apple TV. Ironically, iOS 11, Mac OS High Sierra and the new version of TvOS are venturing into H.265 this fall.. so the stars are aligning.
Many content providers still do not offer 4K. Netflix offers it for handful of shows, but you are speaking as if everything is in 4K nowadays and that Apple is years behind, which absolutely isn't true.
Yeah, yeah, yeah. Defense. Defense.
Once again, I'll point out that at the time of the launch of

TV4, just about EVERYTHING
else Apple makes had already embraced 4K... just this ONE thing clung to 1080p. In fact, in the very same session that

TV4 launched, Apple had just touted the incredible 4K capabilities of another Apple product.
It's funny how quick we are to rationalize why this ONE product does NOT have 4K but you don't see such rationale being blasted at Apple for embracing it in all the other products. Does it really make little sense here, in this ONE thing... but makes perfect sense there, in everything else? Or does it make sense there because Apple has chosen to have it there... and it makes no sense here yet because Apple has chosen NOT to have it here yet?
Answer carefully. Conceptually in just a month or two, Apple may roll out a 4K

TV5. If it makes no sense to embrace 4K in this thing today, it probably should still make no sense barely 4 or 8 weeks from now. Seems a passionate argument against it today should have one back ripping into Apple a few weeks from now for embracing it then. Of course, we know that won't happen. Instead, it will be "shut up and take my money."
And it wasn't about Apple being on the cutting edge. They were pretty much already LAST at that point too. Of the major players, who did NOT have a 4K STB at the time?
Furthermore, Apple had already rationalized h.265 BEFORE

TV4 as the FaceTime codec. Apparently, the "cost benefit analysis" and stars aligning justified it being used there?
And no, there is not a ton of 4K content and I'm certainly not remotely implying 4K is everywhere. As I've shared many times before on this topic,
hardware must lead. Put a bunch of 8K STBs in homes and some Studio will be tempted to roll out some 8K to see if they can make a profit. If they do, more 8K will quickly follow. It never makes sense for software products to be everywhere before there is much hardware on which that software can play. If we wish to make the argument that software must be everywhere first, there's not a single app in the iPhone app store yet exclusively for iPhone 8's capabilities, so perhaps iPhone 8 should wait until "everything" in the iOS store is already iPhone 8 upgraded before they roll out that new hardware? Hardware always leads.
If the software is not quite there at launch, it catches up. It never, NEVER works the other way.