There is no good reason for sticking with 6V batteries.
However, I agree with Jaydee325. The real question is, why are his 6V batteries only lasting a year? Time for some trouble shooting as to what is going on here. Is there a drain on the batteries when not in use, or is the tractor unused for extended periods of time? (all lead-acid batteries self-drain over time, and the process is accelerated in warmer temperatures.)
In addition, I'd hook the batteries up to a battery maintainer when not in use, to prolong life. I've had very good experience with the
BatteryMinder brand. You can buy direct, or find them at Northern Tool: BatteryMinder
model 1500 for a lower-cost model (1.5 Amp, desulfating, temperature compensated), or
model 128CEC1 for a more deluxe model (selectable 2/4/8 amp charging, desulfating, temperature compensated, charge profiles for Gel, flooded lead-acid, and specialty AGM). They also make solar powered charger/maintainers, if the tractor is stored in a location with no power. While their cheapest model, the 12117 has been mostly discontinued (now only sold through Northern Tool) does a decent job and includes desulfating, I don't recommend it often since it does not include temperature compensation - which can be an issue of your tractor is stored in hotter or colder climates). There are other good brands, this is just the one I'm most familiar with.
They make a 6 volt charger, but that's not the way I would be going with this.