Why does Buddhism seem to have more than one goal? Why do some Buddhists regard nirvana as an important goal while other want better rebirth?

Do all Buddhists have the same goal and if not why?
Traditional Mahayana answer is: because of different "capacities". People of "lower capacity" only care about fulfilling basic desires, perhaps just the animal instincts, - then going up the ladder we get those that want material happiness in this world (e.g. comfortable family life), then those who aspire to greater immaterial (but still this-worldly) achievements and realizations, and then those who aspire to rebirth in a better world.

Then we get to the next level, that of spiritual practitioners, who aspire to find the truth, become saints, attain supernatural abilities etc. like e.g. immortality, then those who strive for Enlightenment/Nirvana (this is considered the middling capacity) - and then, according to Mahayana, the higher capacity, which entails desire to attain Enlightenment in order to help all the rest of sentient beings. Finally, in Vajrayana, we get to people of "highest capacity" who directly see Great Perfection of Everything As Is and do not have any goals or desires.

This is the traditional answer (retold in casual simplification). Now if you ask my personal opinion, I would beg you to notice that all these goals are not really in logical opposition with each other. If you think about it, all goals represent various visions of Happiness, or "The Way Things Should Be" - as it appears to each respective type of person. It's not that the lower levels are wrong about their idea of happiness, it's just that they don't quite realize how much hidden suffering is inherent to success on each of the levels.

So really what Buddhism is after, is realization of one's highest aspirations. And on every level of this pyramid Buddha-Dharma provides very rational, methodical, consistent framework for achieving success. What makes Buddhism cool though, is that the practices for achieving success on every level are not just in no conflict with other levels but actually help the levels above and below it.

For example, if one practices emotional intelligence, one will not only have greater control over events of one's life, but also become able to help others still overwhelmed by stress, and also get a step closer to Enlightenment through understanding the nature of mind-made phenomena.

So, to summarize, it seems like Buddhism has multiple goals - but that's just the way it looks from the perspective of different individuals. In reality there is One Goal ("peace"/"happiness"), and Buddhism is the rational method for achieving it.