Hello again Chaski
Very good question sir. Rueben of course was Israels (Jacobs) firstborn, which might seem appropriate that the Messiah come through that line. Another line of thinking would be that Jacob loved Rachel more than Leah, and that Joseph might be the logical candidate, who was a favored son by Jacob. God used him extensively in moving His people to Egypt.
The expansion of the Israelites and their divisions get quite extensive, 10 tribes stuck together while the other two seemed to break off from them. The prophecy was clear however on God's choice in whom the Messiah would come through: (Genesis 49:9, 10) Judah is a lion cub. From the prey, my son, you will certainly go up. He has crouched down and stretched himself out like a lion, and like a lion, who dares rouse him? 10 The scepter will not depart from Judah, neither the commander’s staff from between his feet, until Shiʹloh comes, and to him the obedience of the peoples will belong.
(Hebrews 7:14) For it is clear that our Lord has descended from Judah, yet Moses said nothing about priests coming from that tribe.
Thanks again sir for the great question, I hope I have been of some help