THe usage of מורשה as heritage differs from the usage of ירושה based on what the person inheriting the property is allowed to do with it as well as how it is handed down generation to generation. A heritage is similar to the English law of entailed property in which the property must be passed down through the heirs and may not be sold or given to anyone else.
This is why the halachah of יובל (Yovel -Jubilee year) requires that all land in Eretz Yisrael must return to their original owners. Similarly, the Torah was given by Hashem to Bnai Yisrael in the same way and is something that they cannot give up or abandon under any circumstances. It is what makes us a nation and we cannot exist without it, nor can it ever be taken away from us.
Art Scroll points out in the name of Rabbi Mordechai Gifter (V'zos Habracha 33:4):
An inheritance belongs to the heirs to use and dispose of as they
please. A heritage, however, is the property of generations before and
after; it is incumbent on the heirs to preserve it intact.
Similarly, it is not just that the land was purchased or conquered or obtained by Bnai Yisrael in the normal way that land is transferred in the rest of the world. It is the creation of Hashem who kept the ownership and made it the *property of Bnai Yisrael in the same way that He gave them the Torah at Sinai. In both cases, they cannot give it up or abandon it. They must continue ownership and live according to the way that Hashem has decreed.
Just as the Torah cannot be taken over by any of the goyim, so too the land of Israel cannot be taken over and possessed by any goyim no matter what they might pretend.
Art Scroll also cites Pesachim 49b and Drashos Maharal that the term מורשה is homiletically related to מְאוֹרָשָׂה married in that the Torah is the property of all of Bnai Yisrael and is the property of the entire nation. Thus it can never be torn away by any group (even among Bnai Yisrael). Similarly, the land, even though divided among the tribes, is also the property of the entire nation and can never be taken over, even if others may (temporarily) conquer it.