As a bodhisattva monk, Amitabha saw that there are unlimited heavenly realms for enlightened beings, but not for those who were still trying to get to enlightenment.
So Amitabha created Sukhavati as a kind of half-way stop for those who aspired to enlightenment and had attained some spiritual advancement, but had not yet reached enlightenment.
For them, Sukhavati offered a congenial place where they could continue to cultivate Buddhist merit.