The first and most important reason is that previous encryptions are increasingly easier to break and counterfeit. This reason alone has generally not been significant enough to force the industry to shift to SHA-2 compliance. However, many browser makers and payment gateways are beginning to require SHA-256 compliance.
By December 2015, Google Chrome users will begin seeing increasingly severe warnings next to domains encrypted with only SHA-1 encryption (The most recent level of encryption prior to SHA-2):
In addition to the initiatives taken by Google, PayPal will be updating their certificates and requiring users of their services, such as Instant Payment Notification, to connect to their servers from SHA-256 compliant servers.
This is a trend that will continue as more services upgrade their security to provide reliable and safe experiences on the internet. It’s advised to ensure that if you’re serving encrypted content that you are using up to date encryption methods.