Because it’s a smaller area than 7x7.
If you consider the regular packing in an infinite plane, tri/hex packing is the most space efficient (least wasted space), so I’d assume larger packings would tend towards that. But in smaller packings, the efficiency loss from the extra size needed to offset the circles outweighs the efficiency gained by hex packing.
7x7 is the boundary where those efficiency tradeoffs switch.
I think you skipped a row.
Also, 6*6+7=???