It's usually best not to base a card on a best-case scenario.
I think of this scenario:
Turn one: Mox, Tropical, Dryad, go.
Opponent does something: Force of Will, 2/2 Dryad.
Turn Two: Land, Cloud of faries, Spiketail hatchling, Standstill, 5/5 Dryad, Swing.
This of course, is impossible mana-wise (you're getting four mana out of three sources) and card wise (takes 9 out of 8 cards), and represents a perfect hand anyway - especially considering Fish can only run two moxes maximum. Regardless Dryad does not ass a consistent threat to the deck, also bearing in mind that the gameplan of Fish revolves around swarming the opponent with multiple smaller threats - not one big one.
Imagine drawing a Dryad late game or a green source you inserted to support the Dryad instead of a red one. You get the idea.