i think timing is one reason but because most women who go to salons already use some type of process on their hair (be it bleaching, dying, perming, relaxing, etc.,) or have in the past and henna really isn't recommended for use by those who have previously processed their hair (and most certainly not when they've done it recent to the time they are trying to get hennaed) because it could damge the hair if not used carefully....
also when they use regular color, they are guaranteed a client every 6-8 weeks...when they have clients who henna they can come in for a re-touch anywhere from 2 weeks to 6 months...so they aren't guaranteed that cash in their pocket like they are with clients who use chemical coloring....
also i think the appearance of henna wouldn't be favorable to a lot of customers, even the smell to some people...so i'm thinking that could have something to do with it...i actually like the smell and the appearance doesn't bother me at all! but i agree!!!!