Ruminants digest in big liquid vat at the front of the process, transit very slow from intake to arriving post-rumen. Horses mainly digest in the hindgut, transit fairly fast to that point, and the feed needs to have its moisture with it when it gets there.
So a feed which swells over time, fed dry, will cause a serious issue with a horse but probably won't with a ruminant - so long as it's fed in moderate quantities and there is always drinking water available.
Plus, if it all does go wrong, there are many good and effective home remedies for bloat in ruminants, whereas an impacted colon in a horse nearly always needs the vet, and pronto.