Here's why:
* Gel polish is cured under a UV lamp, hardening it. Regular nail polish remover is designed to dissolve the solvents in regular nail polish, but it can't break down the hardened gel polish.
* Gel polish remover is specifically formulated to remove gel polish. It contains stronger solvents that can break down the hardened gel.
Trying to remove gel polish with regular nail polish remover will likely damage your nails and won't actually remove the gel polish.
Always use a gel polish remover specifically designed for that purpose.