Because when it comes to online money handling, people are very serious and cautious about their money, if not scared. Therefore they want to make sure that, their money is in secure hands. People also want to see a stable company. So when the company changes their identity often (especially if the new logo looks worse than the previous one) it might send a wrong message to its customers. People can get the perception that the company is not reliable.
Therefore corporate identity change is a tricky business, especially for online money transfer sites like paypal.
PayPal has redesigned its logo and is auctioning off its old logo for charity. The auction is actually for the large sign that had hung in the lobby of the PayPal offices in San Jose. One-hundred percent of the final sale price will support Natural Resources Defense Council