https://www.ou.org/blog/is-aliyah-a-mitzvah/
Rav Moshe Feinstein zt”l (Igros Moshe E.H. 102) discusses whether
there is a mitzvah to make aliyah and why many rabbanim have lived in
the Diaspora. He writes that even according to the opinion of the
Ramban, that there exists a biblical obligation, the nature of this
obligation is different than many other mitzvos. He feels that this
mitzvah is “kiyumis” not “chiyuvis”.
To fully appreciate this distinction, it is necessary to offer an
introduction regarding different types of mitzvos. There exist two types
of mitzvos: 1) “kiyumis”- A mitzvah that is not obligatory, rather if
one fulfills this mitzvah one receives reward (ex. Eating in the Succah
following the first night. For the following meals one does not need to
eat in the Succah, if one were to choose, one can refrain from eating
bread and the like and would not be in violation of any prohibition.
However, if one chooses to eat bread then one must eat it in the succah.
Thus this mitzvah is “optional”.) 2) “Chiyuvis”- A mitzvah that one
must fulfill and is obligatory in nature. (ex. Wearing Tefillin. One
must wear tefillin every day and if one neglects this mitzvah and does
not wear tefillin has done something wrong. Thus this mitzvah is
“obligatory”.)
Now we can understand the ruling of Rav Moshe. He explains that
although there is a mitzvah to live in Israel, however, this mitzvah is
“optional”. Meaning that one is not obligated to move to Israel, rather,
if one lives in Israel he fulfills a mitzvah. Rav Moshe continues, that
because there is no prohibition of living outside of Israel it might be
preferable to do so for the reasons given by Rav Chaim Kohen (namely
that there are many mitzvos and prohibitions that apply specifically to
the Land of Israel and it is difficult to fulfill all of those
obligations.)
Rav Yosef Dov Soloveitchik zt”l agreed with the premise of Rav Moshe
Feinstein. He also felt that the mitzvah to live in Israel is a mitzvah
“kiyumis” and not “chiyuvis” (oral ruling cited by Rav Hershel Schachter
shlit”a sefer Peninei Harav).
