Twelve astronauts walked on the moon in six separate missions. Eleven Hasselblads were left on the moon.
"Eleven?" you ask.
Yep. When Neil Armstrong and Buzz Aldrin* walked on the moon as the first humans to do so, they had one camera between them, and it was strapped to Neil's chest. The classic photo of an astronaut from that first landing that has been incorrectly attributed as Armstrong is actually Buzz.
For each of the five missions that followed, each member of that mission was equipped with a camera, which is why there are eleven cameras left behind.
As to "why," it's due to weight reasons.
The cameras themselves were modified versions of the 500EL/M (?) in Hassy's product lineup. In addition to protections against the vacuum of space, radiation, etc., things like the mirror were removed to save weight.
Incidentally, of the moon-modded Hasselblads, at least two never made it there: they were assigned to Apollo 13. I don't know what became of them, but I will also presume that the cameras meant for Apollo flights 18, 19, and 20 are still extant, as those programs were cancelled due to budget cuts.
Dammit, I just HATE when I have a bout of encephalatic flatulence!
I meant to add, as an asterisked postscript, that my girlfriend met Buzz Aldrin when she worked at Walt Disney World.