The explanation could have something to do with the evident sweetness of temper of "Tommy" Hobbes (as my old poli-sci prof at UMSL called him), who envisioned Leviathan but exemplified something more like the lamb in his personal conduct and bearing. Simon Critchley's Book of Dead Philosophers offers an endearing glimpse of a true English eccentric. He "avoided excess 'as to wine and women' and stopped drinking at age sixty," he "walked vigorously every day to work up a sweat... and expel any excessive moisture," he sang "prick-songs" late at night to stimulate his lungs and lengthen his life.
My favorite thing about Hobbes remains, naturally, his peripatetic nature. He walked to work up a sweat but also to stimulate ideas, which he'd interrupt himself long enough to record by disengaging the quill from his walking stick. "He walked much and contemplated," says Aubrey's Life, "and he had in the head of his cane a pen and ink-horn, carried always a note-book in his pocket, and as soon as a thought darted, he presently entered it into his book, or otherwise he might perhaps have lost it."
Another explanation of the failure of "Hobbesian" to convey the menace it might is, of course, a certain sweet-natured cartoonish tiger-cat who resisted his namesake's "war of all against all."