Because China was a poor country back then but had the potential to become a huge market. China was given all sorts of advantages to allow it to develop its economy on the hope that western companies could do business there. China is now a fully developed country (well, the cities are at least), yet it still retains the same economic advantages but market access is still a carrot they dangle before western companies.

Because it was actually a very good deal for the West, contrary to all the gnashing of teeth one hears.

China had to carry out massive economic reforms in order to join the WTO, such as the privatization or splitting up of many state-owned enterprises, large reductions in tariffs, opening up of many sectors to foreign investment, the creation of a legal system to protect IP, and much more. Moreover, China joined under unusual terms that allowed other countries to more easily retaliate against China - sort of a probationary status.

Western companies did extremely well in this relationship. Much of the growth in the West over the past few decades is thanks to China's opening-up.