It's easier to go to Walmart and get a MoneyGram money order. I would imagine that is the biggest competition. I'm also pretty sure Walmart is cheaper.
Walmart money orders are about $1 cheaper than USPS money orders, but there are geographies where USPS is more convenient than Walmart for purchasing or cashing said money orders (my local post office is closer than a Walmart, for example).
Also, USPS money orders never expire. Those from other providers start to incur fees after a duration of time.
Wal-mart often also has bilingual employees and the post office may not, in my experience.
unless you live in AZ.
You got to bring in a DNA test of your first born, social, DL, state occupancy, address, etc etc