I would presume a major issue would be control. Look at how a individual deals with an EMT and consider how large Marstar is versus other vendors
You sell an item, buyer sends you an EMT with password. You click the link and select your bank and the bank asks you to login with user/pass to your bank account to accept it. Then its deposited
The person who processes the EMT will need the login credentials of the bank account, so they not only can process EMT, but they can spend and transfer money with that online access. No business wants a general employee or order processor to have that kind of access to the financials. The controller or accountant has that high level access to the financials, but they don't have the time to run transactions, there job is to review, audit then do the AP/AR for the business. So its a gap between the order processor and the financial controller
Smaller vendors, the owner is often the person processing orders and running the financials or with a small trusted team (such as family members) so they are more likely to accept EMT
Just my thought, but EMT is convenient that's for sure