I'm going to have to agree with the location and supply and demand theories...
in Mesa (AZ; my hometown), acid is the hardest to find. Pot is in pretty short supply lately, too. However, I could find meth or coke in about 5 minutes. If I mosey on over Tempe (the next city over), though I can find acid and pot up the wazoo, while meth and coke are virtually non-existent. (On the meth and coke bit, FUCK THOSE DRUGS!)