I got no idea why Resident Evil 6 has not done so well.

Unfortunate. The actual game was a mess. Selling fans [who are still around] on other game styles that Resident Evil influenced ... as opposed to just being a better version of RE5.

Resident Evil metaC Scores:

Resident Evil 6

X360 (67), PS3 (74)

Resident Evil Revelations

3DS (82)

Resident Evil Operation Raccoon City

X360, Ps3 (52), PC (48)

The resident Evil series has been shit for a while now, it's about time peoples woke up.

Well they ran out of story. RE spent the first few games trying to delve into the origin of the outbreak. So the reason many regard Code Veronica as one of the best is because it finally revealed all the details of the origin of Umbrella and who was behind the virus.
The controls (pre 4 vs after) have always been a big debate point as well. Personally I liked the fixed camera angles. As mentioned above, it added to the "horror". But many people HATED that control scheme, tank controls I believe was the term used.
So you have a game that no longer has anywhere to go, story-wise, because all they can do is have outbreaks in other areas of the world. And since 4 has grow more arcadey with each new game which takes the "survival horror" out of the game completely and instead relies on new, bigger, more unstoppable enemies.
Plus they really mucked it up by releasing too many games. RE3, RE0 were really forgettable. And now there are plenty of me too games like Silent Hill and AitD...not that they were better, only that RE was not the only game in town.
I imagine this was bound to happen but it makes me sad. My first experience with RE1 is still something I talk about. There was nothing like it at the time and still remains the only game to ever make me literally jump while playing it.

Until Capcom get a visionary like Shinji Mikami on the RE team, the series is doomed.
Once that happens, Resident Evil needs to be rebooted fresh start and fresh ideas.

Here's the problem. Tear out the character names, and the plot of Resident Evil 6. Now does the game stand on it's own? Mario can easily stand on gameplay. I disagree that RE6 could do that. Stop listening to focus groups, stop fishing for nostalgia, stop fishing for franchises and just make an original, fun game that your team is capable of making. You did this many times in the past Capcom.

When it's a big franchise like RE poor sales are usually the mistakes made 1 or 2 games previously catching up. It takes a while for the fan loyalty to die out. Not to say that RE6 is a good game, it isn't.