Mitzvos aseh seem to have different levels of chiyuv and 'exemptions'. In general, mitzvos aseh do not require one to spend an exorbitant amount of money to fulfill. Certain mitzvos have particular parameters for being fulfilled that can exempt a person when they are absent.
For example, there is a mitzvah to eat and sleep in the sukkah on sukkos. Yet if it is too hot or cold or its raining, one is exempt. One is also exempt if he is travelling or caring for the sick. The Gemara says a general principle of " 'teshvu'-k'ein taderu" - i.e. one must dwell in the sukkah like one lives in a house. In many cases, one wouldn't live in a house in such a case, so one need not stay in the sukkah either.
Similarly, the Gemara mentions different reasons that permit someone to leave eretz yisrael - to learn torah, for parnassah, or to find a wife. Perhaps one can also say about Yishuv eretz Yisrael " v'yashavtem -'k'ein yashuvu" that one does not need to live in Israel if one won't be able to live a normal life there (See R.H.Schachter). This may be even more broad than by Sukkos, since one will be there for more than 7 days, so other factors can come into play. For example, some people may not know anyone there, or feel that they cannot find the right community there. Some may also be worried for security reasons, even if its not actually 'pikuach nefesh' for other cases. Maybe these and other factors can also be exemptions to the mitzvah.
However, a small financial difference would not exempt someone. In addition, not finding the right community could maybe help an individual, but what about the whole community? Why don't large numbers of them move to Israel together? Also, it seems in many cases in halacha and practice that matters remain as they are even if the reasons no longer apply. People may have had legitimate reasons in previous centuries for not moving to Israel, so that became the established practice, even if it nowadays conditions are very different. It is also hard to move to a completely new place, so sometimes people fail to do it, such as the Jews in the time of Ezra. So people need to evaluate if their reasons are legitimate, or its just out of lack of initiative.
For further discussion and reasons, see R.H.S Schachter's essay and R.A.Lebowitz's outline and sources.