See: http://www.press.uchicago.edu/Misc/Chicago/036480.html
One reason may be better legal aid. But also, a claimant who wants to avoid the risk of paying the defendant's fees if he loses can easily do so, by doing what anyone does when they want to avoid risk: taking out insurance (here, against losing the case).
So if you lose, the insurance company pays the costs order instead of you. If you win, you pay the insurance premium out of the costs award you get. It's called ATE (after-the-event) insurance. So you can basically use an insurance company to nullify the loser-pays rule should you choose to.
Cost orders are on a standard basis, so don't change depending on whether a lawyer is working on a conditional fee.