As Dave said .. price,
but there is more to it than that, as far as I know Fiat are the only van maker who produce a chassis specifically for the motorhome market, at one time it was the only front wheel drive van, this made it easy to lengthen or use with an ALKO chassis .. that's why most of the older Hymers have Fiat running gear ..They can also make them cheaper due to the bulk orders by converters..
There have been problems with the new Fiat, water ingress to the engine bay, reverse gear juddering, but the older models seem to be pretty good, engine wise they are as good as any and better than some..
I wouldn't hesitate to buy an older Fiat based
mh, but would be very wary of new one, that said, many owners are overjoyed and as I have no first hand experience I cannot make an objective judgement ..
jim