He was pretty much the opposite of a people-person, and was definitely misanthropic in general, so it's not entirely surprising that he couldn't really convey that sort of sociability and charisma in his characters.
Basically, he didn't really care about his characters as anything other than plot devices to tell the story (and most of the time, he didn't really care about the story as anything other than an excuse to write 27 pages worth of adjectives like cyclopean or eldritch with 1-2 pages of story thrown in to justify it all).
If anything, it's kind of surprising that he was sociable enough to maintain such a robust letter-writing circle in spite of his personality tendencies. But let's be honest, he was pretty much the archetypal Internet message board poster, just eight decades too early.