Mobile-X is cheap and is "pay for what you use"

I don't think there's some other seedy reason - Mobile-X is just the least expensive option I know of right now in the US that can be purchased at retail, so that is probably the main reason

More data etc, lower price.

I checked less than 90 days ago, and Mobile-X was cheaper than any other MVNO with retail availability in the US.

Do you have links to services that cost less, per line/sim? I don't think they exist, especially at retail.

I might then be completely wrong.