Implementing Deterministic Turing Machine
The reason for that is, if you use Depth-First Search (DFS), there exists the probability of the search going through a path that is infinite in length (loop) and you will never get an answer. If you use BFS it's more likely that you will find a path on to an accept state, if a path to one exists.

For example imagine that you have a tree with 100 children on left from the root and the right child of the root you only have 3. If you use DFS you will traverse the 100-deep left branch and later go right, and now instead having 100 branches just imagine you have infinite number of branches.