Seating distribution in passenger aircraft Recently I was checking in to a flight and was asked if I'd like a window or aisle seat as usual and choose a window seat. I was then told that there are no more window seats available but I could get an aisle seat without someone sitting next to me and then just take that window seat. The plane was an ATR-72 so the rows were 2+2 seats.

I know about weight distribution to the front/back but I couldn't come up for a good reason to do this. What could be the reason for not giving me that apparently free window seat right away?
There are certain restrictions due to which the airliner didn't give you the seat even if you were ready to pay extra for it (Depends on the operations policy by the manufacturer and the aircraft).

It's mostly the weight restrictions for the flight to take-off as there might be some obstacle for the flight during take-off(hoarding on the end of runway, climate etc.). If the weight is too much then it will result in a disaster so equal weight distribution is mandatory.

Moreover in the end it's airliner which has to suffer because the seat occupancy get's affected and it has huge losses incurred by not leaving to its capacity.