Because he and Rosberg swapped crews for this season, probably why Rosberg hasn't really had any issues this year.
http://www.autosport.com/news/report.php/id/123317
They only swap some people, in reality someone should be overseeing the work done and finding any faults. If there is a problem with a member of the team then that guy should be fired. It's likely just insanely bad luck. Rosberg really didn't have many problems last year. Hamilton had way more issues in 2014 than Rosberg had in 2015 and 2016 has been terrible for Hamilton also.
One issue is, one unlucky engine failure leads to Hamilton starting down the grid, this means Hamilton has to push the engine much harder to pass people lap after lap while Rosberg is cruising along at the front under no pressure. So one engine failure leads to a race where Hamilton is in top engine mode pushing hard and running the engine hotter while Rosberg is in 2 engine modes lower, cooler clear air and saving the engine. This leads to another engine going earlier for Hamilton which leads to him having to push the next engine harder again, etc.
Also don't forget Merc had more failures than most other teams in both 2014/2015(other merc teams) because ultimately the other cars were much slower, they don't have the grip to floor it through a lot of corners so lift off while the Mercs are flat out through more corners, higher percentage of each race at the limit.
Then you have Hamilton and Rosberg themselves, same piece of track pushing each other where Massa and Bottas might be 40 seconds apart, so where they both settle for whatever position they are in and save their engines rosberg and Hamilton are within 2 seconds of each other not letting either of them turn down the engine and save the engine.
Mercs by way of being fast and pushing each other use their engines significantly harder than the other Merc teams. It could be Rosberg's car would pushing 1% harder have broken 3-4 engines also and Hamilton pushing just that bit harder is over that 1%.
What we don't know is, have the FIA tweaked the rule they implemented to save Mclaren's embarrassment. Can Hamilton take 2 or 3 engines in a single weekend and start from the back? Also, can he do something to accidentally take his 3rd reprimand, like passing a line on pit exit in qualifying, and take a 5 grid penalty when he's starting dead last anyway. If so he could potentially wrap a bunch of penalties into one race, which gives him a chance.
THis is why earlier when the gap was closing on 50points it looked so bad for Hamilton because then it was obvious he'd take at least one more start from the back of the grid if not more and a likely reprimand penalty as well.
