Here's why:

* Large-scale extraction: Limestone deposits are often extensive and occur near the surface, making open-pit mining the most efficient method for extracting large quantities.

* Cost-effectiveness: Open-pit mining is generally cheaper than underground mining because it requires less infrastructure and labor.

* Safety: Open-pit mines are generally considered safer than underground mines, as there is less risk of cave-ins or other hazards.

However, limestone can also be mined using other methods depending on the location and nature of the deposit, including:

* Underground mining: For deeper or smaller deposits, underground mining might be used.

* Quarrying: This method is used for smaller-scale limestone extraction, typically for construction purposes.

So, while open-pit mining is a common method for extracting limestone, it's not the only one.