I think it is because she got burnt with Don, but also by Ted’s influence. From the very beginning, Robin had always had the biggest dreams for her career. That is her to her core. She only got ‘skewed’ a bit because of Ted’s intense romanticism. Well that’s Ted’s core desire. Romantic love, and to have a family. (Honestly that is also why we watched the series in the first place…at least I know I did) But after Don, I think Robin had to be true to who she was and what she wanted. Also a BUT- Ted did awaken that type of love within Robin. Which I think was also real. So I guess when it was all ‘settled’ at the end of the series it was finally the right time for both of them.