Not because they use different technology. Metro PCS uses CDMA which does not require a SIM card like AT&T and TMobile do. Verizon Wireless uses the same technology as Metro PCS.
If you really want to have an iPhone
and NOT have AT&T, you have to buy your iPhone on eBay and have it jailbreak and unlocked. Then, you could use virtually any service provider you want except AT&T because they will charge you for data. There's no way around it with AT&T.