Similar to some of the other ideas, but here are my framings:

1. Virtually all of the space in the universe have been taken over by superintelligences. We find ourselves observing the universe from one of these rare areas because it would be impossible for us to exist in one of the colonized areas. Thus, it shouldn't be too surprising that our little area of non-colonization is just now popping out a new superintelligence. The most likely outcome for an intelligent species is to watch the area around them become colonized while they cannot develop fast enough to catch up.

2. A dyson-sphere level intelligence knows basically everything. There is a limit to knowledge and power that can be approached. Once a species has achieved a certain level of power it simply doesn't need to continue expanding in order to guarantee its safety and the fulfillment of its values. Continued expansion has diminishing returns and it has other values or goals that counterbalance any tiny desire to continue expanding.