Why is EUR/USD the best pair for beginners to trade?

Because eurusd has the lowest trading fees of all instruments in the market. Remember that as a beginner you are bound to make lots of mistakes when trading and if you go for an instrument with high fees, you account balance can be wiped out. You need to trade a pair that will leave enough money in your account for long enough to enable you get good at this.

Another reason is because brokers allow you use the highest leverage they offer when trading eurusd so this makes it affordable & easy to open a position with little money.

eurusd is indeed the best pair for a beginner trader because when trading the smallest lot size of 0.01 lot, 1 pip movement is will lead to a profit or loss of just $0.1

When trading other instruments with the same 0.1 lot size, 1 pip movement will lead to a profit or loss greater than $0.1 which makes it more risky.