Mind you, I'm not Amish, but do have friends that were / are.
The whole point of the Amish and their Steel wheels was originally twofold. For starters, they used to be cheaper than rubber. Since the Amish were trying to keep their church members from being frivlous, this was a benefit.
The second one is more to the point today. Since steel wheels are much more expensive (from what I've heard), this is really the only remaining reason. According to the Amish church, vehicles which provide transportation are not to be used by any of their members. Therefore, by forcing the steel wheel rule, it ensures that the tractors cannot drive on the road and therefore, cannot be used as transportation -- only for farm work. (This is the same reason that certian Amish churches allow bikes, but without pedals. Yes, they are a form of transportation, but they do not allow people to travel far or quickly.)
Granted, these rules have changed some and I'm no expert on the Amish church and its rules, but I believe that is the basic idea.
To try to keep this post on topic, I think you'll find it had to obtain steel wheels and I do believe they are quite pricy.