Cost of customer acquisition.
Cell phone providers, cable TV, and radio and TV satellite providers also do this. In order to lure you into being their customer, companies offer lower rates to new customers.
Insurance companies are doing this, too, but not so obvious. They will offer rates lower than their competitors and then very slowly and without fanfare gradually raise the rate to one higher than their competitor.
Retail stores do this, too, by offering big discounts for opening a charge account. Of course, this is to get you to be their credit customer more than their retail customer.
You can win in this game by always being someone's new customer.