1. They thought there was no one left in the World (**ibn Ezra** and
    **ibn Kaspi**, **Rashi** and **Rashbam**)
 2. **Radak** says they thought no one would marry them since they came from the destroyed cities they would be outcasts
 3. **Seforno** has the most intriguing answer -- they held themselves to have a great Yichus and there was no one good enough left to marry.