While not the cheapest, IPAs are relatively easy to make and extremely easy to iterate on. IPAs in general allow brewers to fine-tune flavors and thus pump out multiple novel flavors quickly in order to find a market. If you go the stout or lager route, there's really only so much wiggle room as they're mostly 'solved' beers; as in buyers know exactly what they want to taste, and you better deliver that taste. IPAs are also really, really easy to dial in alcohol content without giving up flavor, where as lagers like Budweiser can only lower alcohol content while lowering the overall taste profile, hence the term 'piss water' for low alcohol lagers.