Just because less people play it, doesn't mean it isn't better.
Now, I haven't played both, so I cannot say which is better (I only have Halo 4 since it was a birthday gift, $ issue, not a lack of CoD fandom, I've been playing since 4), but there are many cases where a great game doesn't sell as well as it could.
"Gary was here. Ash is a loser!" = Greatest Tramp Stamp Ever