I figure the simplest solution is the awnser.

she DOES have worshipers, but they're so few and far inbetween that they're impossibly rare. and they can't become clerics under her, so the word of Hel is almost never spread, making converting new worshipers impossible. She can really only rely on individuals making a conscious decision to worship her due to events in their individual lives, and can't count on churches or missionaries to convert people en mass for her.