is there any reason to use reduce instead of map?

It's because StakeProgram.deactivate returns a list of instructions instead of a single instruction. In other words the type system doesn't guarantee that each element in the stakeAccounts list will map to a single instruction (although in practice StakeProgram.deactivate returns only one instruction I believe). Rather than return a list of list of instructions, this method will return a list that's the concatenation of all the instruction lists.

ah got it, in general in those cases it's a bit more idiomatic to use flatMap than reduce