Java 6 was released in 2006. The first dual-core Opterons and Xeons came out in 2005. The licensing for Java 6 was written in a reality where multi-core wasn't really a thing yet.

By the time Java 7 came out, you could easily use a single-socket multi-core processor for workloads where you'd have used a dual or quad socket before. It makes sense that the pricing was adjusted to match the new reality.