It is permissible to pray in clothes in which one has slept, so long as they are pure and cover the `Awrah, although it is better and more perfect to wear beautiful clothes that look good for prayer.

Why Muslims pray 5 times a day falls into the rules of Shari`ah which are purely worship, the wisdom behind which is concealed from us and is not mentioned in either the Quran or the Sunnah. So we have to submit to the command of Allah.