On one hand, I think herbal teas are unpopular due to the stigma it has received in popular culture. It is often associated with "yuppie" culture, and in general, is so often connected with femininity which, of course, turns away a male consumer audience.
On the other, people, especially many tea drinkers, forget that camellia sinensis /is/ an herb by virtue of its botanical classification.
I happen to like tisanes, especially spearmint and chamomile. When I was ill, my grandmother would give me an infusion of sassafras mixed with wintergreen. Though, in my opinion, there's a time and place for them, in every season.