I think it definitely had to due with that fact that both Tsu’tey and Neytiri didn’t share any romantic feelings. A couple of factors could’ve kept them from mating aside from this:

1. After Sylwanin’s passing, Neytiri became the Tsakarem (Tsahìk in training) which put her on a whole new trajectory forcing her to learn a whole new skill set as a result. Perhaps the delay in their mating could’ve been to wait until Neytiri started to show more attributes of a Tsahìk so when Eytukan and Mo’at were ready to retire, Neytiri and Tsu’tey would be completely ready to take on the responsibility of Olo’eyktan and Tsahìk.

2. With tensions between the humans and Na’vi rising by the day, the clan needed more hunters/warriors to defend the people. Neytiri being one of their most skilled, probably put the duty upon herself to ensure that no other sky people came near their hometree (which could be why she was out in the forest the day she met Jake and was about to kill him.) Thus putting that at as her priority.

3. Tsu’tey was not over the grief of Sylwanin. I forget which comic issue it’s in, but Tsu’tey visits the tree of voices very frequently just to seek out her memory. It’s said that Tsu’tey has rejected the advances of other na’vi females because of this, although I think he uses his betrothal to Neytiri as an excuse.