Brand Recognition: Which implies Trust and Network Outreach.
Both Western Union and PayPal are large brands. Their brand signifies that a lot of people are well-versed with the name and procedures associated in sending/receiving money with these brands.
There may be inexpensive alternatives, but that doesn’t mean, they are excellent as far as network outreach is concerned, providing better customer service, having a larger number of point of presence from where money is exchanged.
Essentially, more trusted traction amongst buyers and sellers. Until the competition snowballs into a larger traction base, inexpensive or even inexpensive and most efficient payment networks, won’t be able to compete because no one knows of them or uses them.