Because the trolls are very good at what they do. They usually don't sue, but rather threaten to sue unless the company they're targeting pays a fee. Said fee is set to be below what it would cost the company to fight back.

Virtually every time a company has fought back (and a decision has been reached)[0], the troll has lost. However, it costs companies (especially startups) a LOT more money to fight back. Many roll over and pay instead of fighting, and the trolls make their money.

There is a new law in the works [1] that aims to make the trolls pay the bills in the case the company fights back and wins. This will make trolling very expensive, now that companies can fight without worry of being shut down due to fees.

[0] Sadly, many companies run out of money before this point. [1] http://www.govtrack.us/congress/bills/113/hr845