The problem is that very difference. Halo is a shooter that has always took time to improve in. Years even. But people don't just stick to one game anymore. People are used to playing Destiny and COD and being able to see the improvement instantly. Halo has never had that.

Halo 3 being the common favourite for casual play echoes this even now. It having such a low skill floor allows people to feel like they're improving a lot, without actually doing so, the same way COD or Destiny does it.

Halo 4 suffered because it had no ceiling to reach. The skill floor was so easy for everyone, that improvement was never felt.

Halo 5 is the opposite, and has such a high skill floor that the average Halo fan just got owned, because they weren't willing to put the time in.