I would assume it is because thru their research on the amount of data people use, it is most likely because the majority of people use between 500 MB and 2 GB of data/month. They have determined a lower tier of data is not needed. Of course, you can't make everyone happy. If they offered the plan you are suggesting, there would be those who are complaining that they don't offer a $10 plan with 100 MB/100 messages. They have chosen the entry point to be 500 MB/unlimited messaging.
Keep in mind, when Share Everything was introduced, there WAS NOT a 500 MB tier. It was added later, after they determined there was a need for it. Quite possibly, if ENOUGH people want a lower tier, one will be added.