The problem with Entei is that it can be worn down by weakness to rocks, alongside Sacred Fire will be out of PP especially against constant Suicune switches into it. Yes, it's insanely difficult to switch into, but a speed hitting 299 isn't that fast and can be revenged by a lot of mons. Also the fact that it has one good set limits its versatility, getting locked into a move and can be revenge killed by the likes of Aerodactyl, Krookodile, Salamence, Pidgeot, Mienshao, Lucario (Jolly), Chandelure (If locked into Sacred Fire/Espeed), and a lot more which have over 86 Base Speed. Furthermore, Bulky waters are really common in the UU metagame, Vaporeon, Suicune, Swampert, Slowking as well as Snorlax can come into it and force it out.
The thing about Entei is that it's like Heracross, but it's immune to burn, has a great priority and a great move that almost always burn. On the other hand, weakness to rocks, having a mediocre speed stat and having one viable set makes it stay in A+ rank.