As others have pointed out, mode 1 probably originated from the days of reed-radios where the sticks were nothing more than a spring-centered switch that could be pushed either side of center and which then drove the servo to its full travel until released.

This meant a whole generation of fliers grew up instinctively using their left thumb for elevators and their right thumb for turn (be it rudder or aileron)

When proportional sets arrived it was only natural that these fliers would continue to use the configuration most familiar to them.

Once dual-axis proportional stick gymbals became the norm, I guess some people decided that model fliers should operate their controls as close to full-size as possible -- hence the roll and pitch controls were moved to a single stick with throttle and yaw on the other.

The reason that mode 1 remains so popular outside the USA is probably down to the cost of radio gear.

My first Futaba 4-channel proportional RC set cost the equivalent of several thousand dollars in today's money and they were like hens-teeth in this part of the world. Those were the days when importing stuff from the USA required that the importer had their own overseas funds and were prepared to pay more than 40% in customs duty.

As a result, lots of people opted for cheaper radios and many of those (like the little plastic Futaba 2-channel sets) used two single-axis sticks. This forced you to fly mode 1 and it was only natural that when transitioning from such a set to a full 4-channel box, the same mode would be retained so as to eliminate the need for retraining one's thumbs.

I fly mode 1 and I like it. As others have pointed out, separating the two primary controls (pitch and roll) means that I never accidentally feed in a little of one when varying the other.

And don't worry about the lack of control using your left thumb -- it's not an issue. A huge number of people drive stick-shift cars on the other side of the road to the US -- which means we have to use our left-hand to change gear. It's not hard and I find that when jumping into a US car that changing gears with my right hand is decidedly awkward.

Horses for courses -- I'm sure Mode 1 fliers have a raft of justifications for their chosen mode, just as Mode 2 (and 3 and 4) users do.

There is no right or wrong -- just whatever makes you happy.