Arguments over syntax aside, there is something to be said for recognizing that a loop that steps only one variable is pretty useless, in any programming language.
It is almost always the case that one variable is used to generate successive values while another is used to accumulate a result. If the loop syntax steps only the generating variable, then the accumulating variable must be stepped “manually” by using assignment statements.. .or some other side effect.
The multiple-variable do loop reflects an essential symmetry between generation and accumulation, allowing iteration to be expressed without explicit side effects.
Steele and Gabriel, “The Evolution of Lisp”