The earth is considered to be a middle planet somewhere between heaven and hell. There are parts of this planet that are quite heavenly and others that are hellish, and it seems impossible to find purely one or the other anywhere. Conditions on earth are good for enlightenment because there is opportunity to satisfy the material necessities of life without having to spend our entire life doing so, yet there is enough suffering to make us want something more.
On the truly hellish planets or in the hellish parts of this planet, persons are suffering too much and have to struggle too much just to exist and therefore have little energy to devote to self realization. On the heavenly planets and in the more heavenly parts of this planet, persons may be too busy enjoying material pleasures to spend a lot of energy seeking enlightenment although the attainment of such a life usually implies a more developed consciousness.
It should be understood that humans are not the pinnacle of the evolutionary process. Some of the demigods and residents of the heavenly abodes are as advanced above us as we are above the monkeys. Others in the hellish realms may have very little developed consciousness. Neither these material heavens nor hells are eternal.
Om shanti,
Steve