It would have made more sense to have had 3'x3' so that its 1 yard. Or 3 yds x 3yds which is 9'x9'. But in old school D&D it was always 10' squares I think to make it easy to count up the number of feet if you cast a spell like say lighning bolt which traveled a certain distance in feet. IMO they should have done all the spells in yards instead.

I think its all imperial because a) D&D was done in the US and enlightenment in that land has taken while to master and b) D&D is fantasy based on middle ages and esp a Tolkienesque world where Tolkien wrote the Hobbit pre metric age too.

And before you all troll me about enlightenment, try going into a bar in the UK and asking for a litre of beer. Not happening. We're very much half and half imperial here too.