I think you're just having bad luck
However, Trapinch, Cacnea and Baltoy all have an equal chance of appearing, as they have 20% encounter rate if you're walking through the desert. Keep searching, as I'm sure you'll find one.
Source 1: I've played Pokemon Sapphire, and it's pretty similar to AS in that respect
Source 2: Serebii
Hope you find a Trapinch! :D