H₂S as a source of electrons for plants
The major reason for this is because H20 can cause hydrogen bonding. Hydrogen bonding is what allows for plants to transport "this electron source" from their roots through their stems an further. 

The stronger polarity of H20 also allows for stronger interactions. Such as ionic interactions (not fully bound but 2 polar atoms interacting). This kind of ionic interaction also allows for other types of material interactions such as cohesion and adhesion.

Overall, aside from the discussion of abundance as mentioned above (which is a very good point), it also allowed the plants to grow in directions that were more advantageous (darwinian) for their survival.