With backtracking you typically have some kind of pruning stragegy. That means keeping track of your best result so far and dont go into branches where it is clear that they can never beat your current winner anymore.

So you cannot simply concatenate the results of the subbranches, each new branch needs to be aware of what has been found so far. That can quickly become ugly in pure FP, that's why people have been looking for solutions that do that in an elegant way, e.g. using Monads.