I think.
HW 2 is a wonderful game,
HALO initially released a bug that was not suitable at first
(Even now, there are save data bugs and freeze bugs at startup)
For that reason, evaluation was dropped by many reviews
343 Industries has problems with quality control.
Quality issues may have been a factor, but the fact remains that RTS isn't as popular as the other genres anymore - especially on console.
Ease of access and complexity are natural barriers of entry for RTS. As the population and popularity decline, these barriers become more apparent.
RTS may appear too complex to the uninitiated and can have a somewhat steep learning curve when compared to other game types. And what makes all this even worse is that due to its limited popularity the community and population are often smaller and populated by a more hardcore fanbase. Casual newcomers will get blown away when they try to enter the online competitive scene if they don't quickly grasp the fundamentals.
Losing isn't fun. Losing and not knowing why you're losing is even less fun. Getting annihilated repeatedly blows chunks. Casual gamers are less likely to stay around, so there are fewer casuals for other casuals to play against, exacerbating the issue.