Why can't a woman travel without a mahram?

Answer

Rasulullah (Sallallahu alayhi wasallam) said that it is not lawful for a woman who believes in Allah and the Last Day to undertake a journey without a mahram. The sin is extremely grave. Travelling without a mahram brings her close to kufr, hence Rasulullah (Sallallahu alayhi wasallam) mentioned 'belief in Allah and the Last Day' in stating the prohibition.