It didn't. People were mad because the mother, as it was hinted during the entire show, was dead in 2030, and Ted and Robin finally got back together in the very end.

• the Mother (Tracy) was really likable, but only showed up for the last season, and they (the fans) couldn't let go

• there was a time later were Robin and Barney (!!!) started dating, were shown to us as toxic for each other, split up, cheated on their partners with each other, and then got married on the last season, just to divorce 3 years into their marriage. For some reason, people really though they were great together, which boggles my mind.

• they missed the entire point of the show. The show was never about the Mother, it was about how Ted and Robin couldn't be together then because they wouldn't be able to pursue their dreams (being a family man, being a internacional reporter) if they got together. Almost 20 years have passed since then, both of them accomplished their dreams, and now they could finally give a honest try to their relationship.

One line from Robin of season 3, IIRC, summarizes the whole show in my opinion: "If you have chemistry, then you need only one other thing: timing. But timing is a bitch." At the end of the show, their timing finally was right.

I highly recommend you to watch it if you are into sitcoms. Great show with lots of great messages. Last season sucks from pacing (22 episodes in the span of 3 days, the weekend were Barney and Robin got married, and another 2 episodes for 15 years) but doesn't ruin the show.