Hay Wilson has good points. 

Some horses crib the krap out of everything if they don't have something to munch on.  And some horses react to alfalfa like kids to candy. Full of 'rocket fuel' and act like it.

Some horses are amazingly delicate. Eat too much alfalfa and/or straw and their either compacted, floundered, or some other crazy krap that some horses seem to get.

Then of course there's some hay buyers who pay more attention and worry more about what their "quality horse" eats than what they or their own children eat. (always chuckle some whem people *roll out* of their rigs to look at hay (all hyper concerned about the feed quality of course) and all I can think of,, *poor horse must be really tuff having to pack that kind of weight around..*

Alfalfa isn't good if the horse is a real *easy keeper* or tends to *bind up* easily.  Some horses don't do well having their feed constantly changed either. (maybe mixing up a few handfulls of straw and alfalfa would be best)

I always marvel at the wild mustang herds,, live by finding their own food, and are pretty dang healthy.  The family 'horse' meanwhile  either 'plugs up' or has problems eating better quality feed than wild horses.