Overload is not the same as in purely extra mana, it's better to have a 1 mana spell with +1 overload than a 2 mana spell.
Overload is powerful when it allows you to borrow tempo from later turns. Make a more powerful play earlier, and weaken a later turn slightly. But Finders Keepers isn't playing the tempo game the same way, so overload is much more of a drawback here than it would be with a card like Totem Golem. There are still benefits to it being an Overload over a straight 2 mana card but the difference is marginal. It's still much closer to a 2 mana card than a 1 mana card in cost.