"Better" is defined by the user. "Popular" may be defined by a forum.
Harts are popular because they are nice. I have a couple and like them, but I have lesser priced razors that shave as well...and I have razors suited to my tastes that I might call "better", but some would not share my preference.
This being said, I believe the steel and heat treat/hardness of the Hart does lend itself to slightly longer edge life compared to some other nice razors I have....but I only have a couple data points to support this;)
Edit to add: if it is not clear, I am saying that I agree...different but no better (or worse). A diplomatic (and very successful) knife maker frequently uses this statement about competitor's offerings: "all good, just different"
:)