Hozier - has seen his song*"Take Me To Church" becoming the biggest sleeper hit of recent times. Originally released in Ireland in September 2013, then released to radio in the US in March 2014, it*slowly climbed - hitting number 2 in the States in December 2014, and ended up dominating the UK airwaves throughout 2015 - taking 26 weeks after first entering the top 40 to hit the top three.
They take forever because not many people know about the song. If someone who knows it performs the song on a talent show or something, it gives the song exposure. Sometimes I think old songs are randomly planted into radio playlists by record executives so it may seem like it blew up through actual promotion. Just like Mike Posner's song currently.
Latch, Here, and Hide Away all took around 6 months to reach their (high) peaks on pop radio due in part to discovery of the songs being slow (as the lead artists were all total unknowns before their hits).
Radioactive took forever to become a colossal smash (because it had to cross over from alternative to HAC to pop), and Like I'm Gonna Lose You waited until winter to pop up (probably because radio PDs didn't think it fit during the summer/fall).
Macarena by Los Del Rio. It was originally released in 1993 and climbed to like #54 and then dropped off the charts. Then, in 1996, it got an even cheesier remix and went on to becoming one of the longest running (14 weeks) and best selling singles in US history.
Another one that took forever to go to #1 was "Sweet Dreams" by Eurythmics. It was a slow burner, but it became a classic.
I remember Counting Stars taking FOREVER to catch on. Radioactive as well, in part because It's Time was slaying when it released and then it had to cross over from various formats. I think Habits took over a year as well