I missed this topic, but I see @Flohack has already provided solid arguments.
elementaryOS is backed by "elementary LLC" which likely enables them to handle money on a solid legal base. I gave a quick look at "Houston", the app store back end used by elementary, and it seems they only support a single service, called Stripe. I haven't tested personally, but what I see is that developers might likely have to create a Stripe account in order to receive donations.
This might add a further consideration: choosing a single service implies excluding others.
We could offer e.g. PayPal support, but I'm mostly sure our developers might (legitimately) ask to support Patreon, Liberapay, Bitcoins, etc. Not to mention potential existing service bans in specific countries. It might end up in a huge amount of work, for an app store service that merely counts 400 apps and has not fully shaped yet.
What we did in OpenStore, but we still need to expose it to the client app, is to let developers to specify a link of their choice for donations. This already enabled developers to received donations through the OpenStore web UI - e.g. Dekko.
I'm in charge to get this done in the client app. Despite my expectations, adding a single button to an existing hiearchy of informations is not so trivial; as a consequence, it might take a while before we could roll this out in a new update, since we have to redefine how we provide other metadata.
I'm currently redesigning the "Package Details" page in the OpenStore client, and the idea is to ensure that our "Donate" button would always be placed above the fold, in order to ensure that potential donors would not miss the opportunity to support the work of developers and contributors.
Here's how the "Donate" button might look in future OpenStore releases