Because the HTC EVO 4G was released on June 4th, 2010... and the Epic 4G was released on August 31st, 2010. The HTC EVO 4G received Android 2.3 on June 4th 2011 (Exactly 1 year following release), so if that's any indication on the Epic, it should receive 2.3 by the end of August at the latest... but based on leaks and rumor, it may be sooner than that.
Also noteworthy that they both shipped with Android 2.1 when they were released, and the EVO received 2 OS updates in 1 year, and it looks like the same will happen with the Epic, right on schedule.