Isinglass is not used very much outside breweries trying to do really traditional British styles, because it’s finicky and there are better and cheaper options. I’d say that the most common thing making beer non-vegan is adding adjuncts like lactose in milk stouts.
It’s because you have to use your water to keep water rights. Flood irrigating alfalfa is an easy way to use up a lot of water to keep the rights without having to manage more complex farms. The current way that water rights works creates perverse incentives.