Some people here are so desperate to avoid having to admit WotC can do one wrong, they're constructing a narrative where people are retconned into not expecting their animal companions to survive combat, or not to enter combat at all.
We've always been at war with Eurasia!
But seriously, while I wish it were otherwise in this case, it just seem that WotC's idea of "wrong" is very different. Rangers are being played, they seem popular (now...), and there's no money in addressing the concerns people have for the class.
This is "greatest hits" D&D; it's not risk-taking, it's not trying to be super-innovative...it's trying to be popular. As a business model, there's nothing wrong with that. Do they think they risk hurting the brand or ever-greenness of it by having multiple sources for the rules in different areas? Probably. After all, when you buy a board game, you expect to be able to play right out of the box, right? Not have to scour online for the other rules?
So even though the PHB Ranger is lacking, it's not lacking enough to make what they feel is a bad marketing decision...and the latter will always be more important than anything else as far as the game goes.
This isn't even the first time this has happened. The sorcerer has been seen to be lacking even moreso than the ranger, and when the UA Storm Sorcerer was released with bonus spells known, it was seen as a well-needed and appreciated fix. Didn't stick, though, since the PHB sorcerer didn't have bonus spells...so even though the fix made sense from a class balance perspective, the fix flew in the face of the evergreen product and so was removed.
They are making decisions so that the game lasts longer, and maximizes profit...some things are going to get lost in that priority scheme, is all.
In the meantime, since 5e is quite forgiving and really doesn't seem to care much about balance, I would use the UA Ranger as is. It's a bit overpowered, sure, but big whoop.