Narrowing the search space greedily doesn't solve the problem optimally, because the size of the search space is just a heuristic. Some search spaces of size X

Narrowing the search space greedily doesn't solve the problem optimally, because the size of the search space is just a heuristic. Some search spaces of size X might be more difficult to reduce further than another search space of size Y (for Y < X). This is more apparent in hard mode where you have a very limited guess-pool. The greedy heuristic approach would work fine if all possible 5-letter combinations were valid answers.