Probably because everyone forgot about Kevin and Robin. I certainly did until you mentioned them again. And it was creepy, you can't date your therapist. That was another bad choice, and Kevin didn't fit in the group nor did he have any chemistry with Robin...
Well I am sorry to say that the owners of the show made it very clear that Barney and Robin WILL make it work cause even though their wedding doesn't go smoothly they DO get married!!!
I think they make an awesome couple, they are very similar and love each other unconditionally, I think they make a perfect couple! Just because the writers screw it up in season 5 and didn't know how to handle their relationship doesn't mean it won't EVER work! At that time Barney and Robin were completely different people, even though they loved each other they still had HUGE issues! They were insecure, Barney still didn't know who his father was and had abandonment issues, Robin also had daddy issues(and still does, I think they should work on that) so they were not ready to commit to each other, their timing was wrong! That doesn't mean they can't make it work now in the contrary I think the show has done a wonderful job in transforming Barney from a playboy to a boy who actually respects women and wants to make his relationship work so he can finally be the ready for marriage! Of course you have your opinion and I have mine, you think it won't work but I am sure it will!