The US Dollar is one of the most popular currencies among forex traders. It is estimated that over 85% of the daily traded volume in the forex market is done in USD.
The popularity of the USD is because it is widely accepted all over the world as the reserve currency. About 60% of the foreign currency reserve by central banks around the world is in USD while Euro accounts for about 27%.
Here are some of the reasons why the USD is popular in the forex market. 
Major Forex pairs are Quoted against the USD 
In the forex market, major currency pairs include currencies from the developed nations quoted against the USD. Forex traders prefer the major currency pairs due to their higher liquidity and low volatility. Some of the major pairs include EUR/USD, GBP/USD, AUD/USD/ USD/CAD, USD/CHF, USD/JPY, and NZD/USD.
Source: https://www.entrepreneurshipsecret.com/why-is-usd-the-most-popular-currency-in-the-forex/