Roughly 90% of the world (according to google) is right-handed, so most gates would have the reader/whatever on the right by default. Off the top of my head I can’t think of any systems that have it on the left.
I’m right handed, but keep tickets in my left pocket, so use my left hand to retrieve them, and hold them. Not a problem if I’m not carrying anything in my right hand, as I can exchange hands.
Otherwise, I have to reach over with my left hand.
I’ve also been too quick with presenting my ticket, so, the person ahead of me gets let through and I get trapped

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Also, if it’s busy, especially there is a queue, if someone is blocking the sensors, or because of the number of people, you yourself may step too far forward and block the sensors yourself, the gates will refuse to open

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The other really annoying problem is if you go out the wrong exit (perfectly possible if visiting a station for the first time, where you don’t know your way around), in most cases, the machine retains your ticket and you then have to go and explain to a member of staff…
Tickets gates, like so many other systems on the railways (and in modern technology) definitely gives me the impression that there has not been enough thought put into them from a human factors point of view.