3 Prevalent Themes
1. Incentive to contribute
"Same reason why you would use arxiv instead of posting the result to X." — mlpoknbji
"Why publish research articles? Why contribute to the Linux kernel? ..." — teiferer
2. Vision of a unified, searchable formalization of mathematics
"Turning the entire field of mathematics into a formalized and connected system... All fields will undergo this change!!!" — seeknotfind
3. Platform dependence & verification difficulty
"However, checking that a given Lean repository actually proves the claimed statement is somewhat non‑trivial, especially for an audience which is not expert in the use of Lean." — demibabs
"It only works for GitHub;" — fuglede_