In addition to referring to a particular tribal group, Canaan was a territory in which multiple people groups lived at that time. Amorites, Moabites, Hivites, Hittites, and more, were all inhabitants of the land of Canaan. Here are some texts in support of this:
Esau took his wives of the daughters of Canaan; Adah the daughter of
Elon the Hittite, and Aholibamah the daughter of Anah the daughter of
Zibeon the Hivite; (Genesis 36:2)
And I have said, I will bring you up out of the affliction of Egypt
unto the land of the Canaanites, and the Hittites, and the Amorites,
and the Perizzites, and the Hivites, and the Jebusites, unto a land
flowing with milk and honey. (Exodus 3:17)
Then the dukes of Edom shall be amazed; the mighty men of Moab,
trembling shall take hold upon them; all the inhabitants of Canaan
shall melt away. (Exodus 15:15)
As can be seen, those verses seem to lump these all together. The word "Canaanites" can have two senses of meaning: 1) those that lived in the land called Canaan; and 2) those descended from Canaan (Ham's son). The latter sense of meaning more properly applies to this next verse:
The Amalekites dwell in the land of the south: and the Hittites, and
the Jebusites, and the Amorites, dwell in the mountains: and the
Canaanites dwell by the sea, and by the coast of Jordan. (Numbers
13:29)
So when Joshua says "Amorites" instead of "Canaanites," he is simply being more specific about which tribal group he is addressing. All Amorites might have been considered Canaanites, but not all Canaanites were Amorites.