Originally Posted by Often1

Those deltas are likely an apples vs. oranges situation. If you are comparing BE to the most restrictive Main Cabin fare, the delta tends to not be that large (although growing). But, in this case, it is possible that the lowest available fare buckets in Main Cabin are not on the lower end because they have been purchased and thus OP is looking at a BE to mid or high-range fare.

There are many businesses (and individuals) who simply will not purchase BE fares. Thus, BE is its own market. Others, like OP, do look at the delta and then have a decision to make on flexibility, seat assignment, and the rest of the features of a ticket.