Alyson has two children and has clearly enjoyed being a stay at home mom. Fair play to her - she had already 'made it' before HIMYM so she can take a few years off to spend with her kids.

Josh seems to be more of an indie guy judging his movies he did while also starring in HIMYM. I bet he had some offers but doesnt want to be that kind of sitcom star to venture into romantic comedys. After 9 seasons of a hit TV-Show I bet he doesnt need the paycheck too bad either.

Alyson was already well known prior to getting this show.

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Right but who cares what she does
after
HIMYM? She was in American Pie and on Buffy and HIMYM. All were very successful endeavors. She can retire today and she would still be considered to have had a very good career.

Yeah, she's such a has been now
He lied to us through song! I hate when people do that."

Make it after HIMYM

Alyson has already made it She was very much well known before HIMYM.

ALyson already done everything she wants. She's been a star for a long time

I don't think it's a case of "suffering", but a matter of want, and whether they feel the desire to pursue other large roles. Alyson went from Buffy almost straight to HIMYM; and between the two shows she has 16 seasons of almost nonstop television work under her belt, plus several films. She seems to be content, at least for now, raising her two young children. She's certainly not hurting for a paycheck.
By some account Josh Radnor was never much for high profile work outside of HIMYM to begin with, and since the show ended that hasn't changed. He had done mainly stage work before landing his role as Ted, with a handful of small exceptions. He's done at least a couple of independent films, one I believe he may have written and directed; and that seems to be the sort of thing he's interested in doing.
You do realize both Josh and Alyson did other projects during HIMYM, and have been doing other things since, including within the last year, right? I mean, you're on IMDB, so I'm assuming you bothered looking at their film history before posting wild assumptions?
He lied to us through song! I hate when people do that."

Jason, Cobie and Neil had other gigs outside of the show while it was running. I guess Josh and Alyson couldnt make it happen.
Josh wrote, directed and starred in two movies while he was was on HIMYM. He was also on Broadway for 6 months after it had finished and did Mercy Street on PBS. He has also written plays and will be in another;
http://www.lct.org/shows/babylon-line/
He is now writing songs with Ben Lee and writing and directing music vidoes starring Alison Janney
https://www.youtube.com/watch?v=hOTR3iSu9OQ&feature=youtu.be
He is obviously doing what he enjoys