From Java Platform, Standard Edition 8 Names and Versions:
In JDK 8 and JRE 8, the version strings are 1.8 and 1.8.0
Why? Because the developers of Java chose to name the versions like this. I can only assume the real reasons, but I think it is, because naming it Java 8 implies that it is new and much better than Java 7 but keeping the version bump from 1.7 to 1.8 indicates that it is still version 1.x and therefore still backwards compatible. In the end it is marketing.
See also Why is Java version 1.X referred to as Java X?.