One huge advantage of the ATmega328P is that it's available in a through hole package which means a beginner can prototype with an Arduino board and then be able to easily solder the same MCU in their finished project. Sure there are faster and cheaper ARMs, but the ATmega328P is sufficient for most Arduino applications. For a hobbyist the price difference really doesn't matter.
There are many other options available out there, including in DIP packages, so if you want something faster with more resources, buy that instead.Use the old "Vote with your Wallet" method.
If you want to buy an Arduino then those options are not on the ballot and voting for the ARM candidate will make a much larger dent in your wallet. So most people will continue to vote AVR.
DIP package are completely obsolete.