To expand on what Eric said, the field has actually changed shape quite a few times, just not in recent years.
Way back in the beginning (1992), the field was square, elevated on a table, and filled with corn.
<Knowledge gap - someone older than me will have to fill this in...>
In 1995 (Ramp n' Roll), the field was a T-shape, with a large ramp/mound at the top of the T where the football goal stood. Hexagon Havoc (1996) introduced the hexagon shape (and also human players). The hexagon stayed for the next few years along with the 1v1v1 format.
With the switch to the 2v2 alliance format in 2000 (Diabolical Dynamics), the field became rectangular, and kept the same dimensions through the following years (Co-opertition FIRST, Zone Zeal, Stack Attack, FIRST Frenzy). Triple Play (2005) was the first game to use alliances of 3 teams and the size of the field increased to the now-familiar 27'x54'.
At this point, FIRST (as well as FiM and other regional groups) have a huge amount invested in the current field setup, so it's unlikely to change. Many venues, such as those held in hockey arenas, have severe space constraints that make it impractical to fit a larger field. As much as I'd like to see something besides 3v3 just to shake things up, the match timing and logistics of the 6-teams-per-round format seem to be nearly optimal.
That said, there are a lot of cool things that could be done with a different field format, many of which have been mentioned before here on CD.
Drivers in the middle of the field, anyone?