The answer is simple. In the USA, only one to a hand full of people at any club, with exceptions, fly Mode 1. If you are learning, or have soloed; sooner or later you will need help, asked for help, or just want to fly someone's plane. If you fly their mode you will be able to fly off of their transmitter. It just makes it simple.

40 plus years ago when I learned to fly, there was the right side and left side of the flight line. I picked the left, they were Mode 1 flyers. With the old radios, (Kraft and others), it was simple to make the change. Newer ones take a little more work.

One good thing about flying Mode 1, my friend (mode 2), and I fly each others stuff all of the time. No one ever knows that I am flying his. It does make some for some laughs when he sets his transmitter on the deck and I take over. It is our wireless Buddy Box.