Our decision to use meeples, custom meeples, and wooden bits was based primarily on a few major factors.
First, real estate and consistency vs. detail. Situations arise in the game that require 3+ characters and enemies on the same 2" square tile, possibly at the same time as other tokens. For this to work, all of the entities would need to be small enough that you would lose most of the detail, or the resource and item bits would need to be a different material.
Second, one of our primary goals was accessibility. Miniatures can be very intimidating to non-gamers, but the word I usually hear when someone first meets a meeple is "cute."