Do you think the popularity of Mercs in those games is down to the expanded melee/movement mechanics?
I think there are probably different factors.
First of all, I think Roy is right and that the insane quality of RE4's main game sort of overshadows all the other modes, for better or for worse. Tone is also a factor, a lot of players are drawn to RE4 due to the slow-paced and oppressive atmosphere of the Village and consider it to be a horror game first and foremost (I disagree with that assessment, but that's how a lot perceive it). But then they often end up disappointed as the action ramps up on the Island and won't be giving much thought to playing an even more action-focused mode afterwards.
Second, RE5/6 Mercs are undeniably more complex with an even higher skill ceiling. I think they lose out on a lot of the fine-tuning, polish, organic play and balance that make RE4 Mercs so remarkable, but there is definitely an even higher incentive to get better at it for the super serious score chasers. Plus the expanded melee system, all the different stages and playable characters... RE5/6 Mercs WR strats are really
complex, even more so than RE4's. A bit like the distinction between NGB and NG2/3RE.
Third, RE5 came out at a time when internet gaming culture was significantly more active than in 2005 when RE4 came out. That plus the coop nature of RE5 in general means that it automatically generated a lot more enthusiasm and activity by default. So the wider Mercs community basically started with RE5. And people generally don't tend to replay older games, so...
To give you an idea, biohazardmercenaries (the main Mercs forum) was always completely centered around RE5, and they even removed their RE4 section entirely during their last layout update.