Why more players perfer WOTLK than MOP in Warmane?

According to related data, now there are usually at least 7-8K+ players on Icecrown and Lordaeron, but rarely over 1,5K players on Frostwolf. Why is that? What attraceted so many people to paly in WOTLK rather than MOP? Which would you perfer or which should you choose? We have summarized some views in the below.