Are people tuning in for all the clues (we meet again, yellow umbrella) and drama (there’s been plenty over the past few seasons), or is it simply because it’s a dependable, no-frills sitcom? It wouldn’t be surprising if the answer is both: HIMYM has no doubt grown in popularity thanks to syndication and word-of-mouth, but there’s just as much can’t-miss cliffhanger drama as there is TV equivalent of comfort food in every episode. (The rise in popularity of stars Neil Patrick Harris and Jason Segel throughout the course of the show probably hasn’t hurt much either.)
I think this has something to do with DVD and streaming catch ups, the show is widely available and easily digestible making it easy to catch up and join in. Few people I know have been watching from the very beginning. I myself came to the show on the fifth episode.