Hi
I can only speak for myself here
The reason why I ship it is because both Braeburn and Soarin are like two of the few men/stallions in the series that I always/actively headcanon as gay and naturally I ship them together because Why not? Sure they in canon have likely never came close to meeting but like Big Mac and Sugar Belle I can Imagine a scenario where they start meeting and eventually fall for eachother. Plus its fun to imagine, two gay horses in love that isn’t women/mares for once.