Couple reasons 1) why use seconds when you can just measure a segment of a known distance? 2) pendulums swing faster or slower depending on altitude, 3) Problems with accurate measurement of seconds, swing time etc.
I guess for (1) I would imagine it was easier for an independent group of scientists to accurately measure a pendulum, than to travel all the way to the north pole in a straight line through Paris.
But I guess they were using the stars or something so maybe it's not as hard as I'd imagine. Also (2) and (3) are great points.
