According to an article by TVWeek.com featuring Futurama Producer/Associate Producer Lee Supercinski, the series were supposed to be produced in High Definition since the first episode back in 1999, but in those times according to the article: "[..] money and technology for HD weren't up to par, and the idea was scrapped".

Today, the four Futurama movies were produced in High Definition thanks to Lee, but in a first moment, FOX didn't want to increase the budget for that to happen.. only when they knew that it could cost only 2% more, FOX said yes.

The changes included only more disk space and the change from 4:3 to 16:9 format. The changes were minimal, but even then "[..] Fox still was skeptical enough to not release the first two movies on Blu-ray, although a HD master tape exists"

As you know, the first 2 Futurama Movies didn't appear in Blu-Ray when the HD master tapes exist. Bender's Game will appear in Blu-Ray, but there's no green light yet for Bender's Big Score or The Beast with a Billion Backs to appear in the HD format.

SOURCE: TVWeek