Why would foster parents be denied to adopt their foster children?

The only reason I can see is if e kids expressed they don't want foster parent to adopt. My boys have a brother and 2 sisters placed in another foster together, their fp's were planning on adopting all 3 but their brother said he wants to be adopted with his brothers (my boys). They let him make that choice at 7 years old, despite the fact he has been with s FP for over 2 years. Their AW wouldn't even consider splitting the boys despite the fact my boys desperately wanted to be adopted by us and we don't have room for all 3. We actually decided we we were NOT going to be able to adopt, but if we had wanted to we would have been denied unless we took their brother too (if it was up to me alone I would have taken all 3, but we had to consider our family dynamics and it just wasn't looking like the right decision)