Payment must be irreversible
A fundamental part of most scams is that payment methods must be irreversible. It must be impossible for the "mark" to clawback the money once the jig is up.
Suppose the scammer took the money via a credit card. They would have to establish a relationship with a card processor, and prove to the processor their creditworthiness as part of that process. Alternately, the processor may force a time delay before releasing the scammer's money. Regardless, once Morty the Mark realizes the score, Morty files a chargeback with the credit card, and the card processor would clawback the money from the scammer. Well, that didn't work.
Or the scammers will deflect through another victim. For instance Laura the Launderer, who answered an add about being an online office manager. In that case, they have Morty send the money to Laura via a reversible method, and then have Laura send the money to them via an irreversible method (and keep 10% as their wage). Morty claws it back from Laura, who can do nothing about it.
Western Union is a workable choice. Another is Bitcoin, and another is gift cards e.g. iTunes. I have also heard about them having people mail cashier's checks to accomplices in homeless shelters: the accomplice cashes the check semi-anonymously at one of those pink check-cashing places.
All the better if you're unfamiliar with the payment method
They prefer to use payment methods you are completely unfamiliar with. If you have no idea what you are doing, you aren't on your guard -- you are relying on them to say what to do, and that gives them more control.