Wow that's a name I haven't heard in awhile. Looking him up he seemed to have a decent run from 2011-2015. No results from last year, and only 3 races in 2018 (best a 13:50-something 5K in Kenya) and only 1 race in 2017 (a dnf).
Also that 12:48 was that super-fast race in Paris from 2012. John Kipcoech ran 12:49 there, and what did he ever do before that or after? Or Paul Longosiwa (12:49 too)? Seems like a few guys simply had the race of their life on that day.
Koech also ran 12:53 indoors in 2011. Got bronze in the 5000 in Moscow, 2013; 5th in 2012 Olympics, and 4th in Beijing, 2015. His last sub-13 was in 2013 and his best DL finish in 2016 was a 4th place in London and he had a 13:08 best that season, so it really has been a long time since he's done much.
I don't know specifically what happened to him, but having a good 5-6 years sounds pretty normal to me. Lots of little-remembered Kenyan (and some Ethiopian) distance runners who only had a good season or two. He did always have sort of a bulky looking build for a 5K guy to me.
What ever happened to Gebremesekel, Alamirew, Horan Keitany, Ndiku, Kwemoi, etc?