top two pair gets beaten regularly.
eg you hold 64 on board K46 other guy has tpgk
then if turn and river are 2s you get screwed. That doesnt happen to top two pair. If i hold top two im not afraid to get it all in if i hold top and bottom pairs on flop K5 in this case im more wary but still likely to get it in. Thats unlikely with bottom two pair.