Always thought it's because 1C is not a very good seat - you feel like your on the corridor sitting in 1C, no wall in front of you, carts bumping on your feet etc. and during boarding you are just on the way in that seat.

I try to get 1A as often as possible as most times 1C will remain empty.
Just picked 1A for upcoming HelLhr flt and yes, 1C is blocked.
on the way back both 1A and C are taken (not blocked), so someone somewhere can release that seat.
.