breadth-first search is optimal if the path cost is a nondecreasing function of the depth of the node. The most common such scenario is that all actions have the same cost.

Why is BFS not optimal?

Optimal as in "produces the optimal path", not "is the fastest algorithm possible". When searching a state space for a path to a goal, DFS may produce a much longer path than BFS. Note that BFS is only optimal when actions are unweighted; if different actions have different weights, you need something like A*.

Which is optimal BFS or DFS?

Answer: If a search algorithm is optimal, then when it finds a solution it finds the best solution. What are the advantages of breadth-first search (BFS) over depth-first search (DFS)? Answer: BFS is complete and optimal, while DFS is not guaranteed to halt when there are loops.