Within numerology specifically, 7's fortunate reputation is usually tied to its association with depth, spirituality, and a kind of intellectual distinction — a number seen as set apart from the more socially oriented digits around it, in a way the tradition frames positively rather than as isolating.
It's worth separating that from the older, non-numerological reputation 7 already carried — seven wonders, seven seas, seven notes in a scale. Numerology's specific contribution is folding an existing cultural favorite into its own digit-based system, more than inventing 7's luck from scratch.