It’s convention, I think. If I remember correctly, you always put y on the left, because you can also write equations as functions of a variable, x, with the symbology f(x) = mx + b. That way you can integrate and derive the function easily, since m and b are constants, and all your x variables are on one side.
If I were to encounter x = my + b, the first thing I would do, just by nature at this point, would be to convert it to y = (x - b) / m.
It’s been a while since I took math, and I was never the best, so others should feel free to correct me.