Why is Mainland China referred to as "大陸" (大陸)?
It's due to the one-China policy (一中原則).

Both CPC (中國共產黨) and KMT (中國國民黨) think there is only one China.
Both refer their own places as countries, PRC and ROC, respectively.
Both treat the other side as an area or region, 台灣地區 and 大陸地區, respectively.
At the beginning, KMT called the other side as 淪陷區.
Because it is not a good term, KMT changed it to 大陸地區.
大陸 is a short form of 大陸地區.
Many people in HK also moved from 大陸 due to the civil war (國共內戰).
Therefore, these people tended to stand on KMT's side, and then used the KMT's term.

中國大陸，中文使用上常通稱 大陸或大陸地區 ，是對中華人民共和國的實際統治領土之稱呼，也是海峽兩岸關係的常見用語之一。 此詞的產生肇因於第二次國共內戰後 ，1949年中華人民共和國成立所導致的海峽兩岸政治分立。
English version