為什麼立法會議員不能提出公共財政的方案?

Because of the Basic Law.

The budgetary authority falls to the Executive Branch under Article 62 (4). The only authority the Legislative Council has is to approve (Note: not deny) the budget only under Article 73 (3).

A council member can make suggestions to the Executive Branch. However, given there is no authority to alter or deny the budget, the Executive Branch basically holds virtually all authorities over budget.

基本法已經規範，只可以由行政會議提出，立法會只是表決通過與否。