Here's why:

* Accurate and Efficient: TXVs are designed to regulate the flow of refrigerant based on the evaporator temperature. This ensures that the right amount of refrigerant is delivered for optimal cooling performance while preventing excessive refrigerant flow and potential damage to the compressor.

* Simplicity and Cost-Effectiveness: TXVs are relatively simple and less expensive compared to other metering devices like capillary tubes or electronic expansion valves. This makes them a suitable choice for small air conditioners where cost and simplicity are key factors.

* Wide Applicability: TXVs are widely used in various types of small air conditioners, including window units, portable units, and some smaller split systems.

While capillary tubes are also found in some small air conditioners, they are generally less common than TXVs due to their limitations in adjusting to varying loads and environmental conditions.

In summary, TXVs are the most widely used metering device in small air conditioners due to their efficiency, accuracy, and cost-effectiveness.