Why the bus always go in left line?

1) 根據香港道路使用者守則，道路左邊線為慢線，因巴士要經常停車，算是較慢的車輛
2) 巴士線在左邊會方便巴士進出巴士站，減少引致交通擠塞
3) 方便巴士乘客上落

也可參考運輸署網頁

because bus line is on the left also the people get into the bus also in left.

because the bus line is on the left
therefore the bus can easier let the people get off or get in to the bus