Many math textbooks and papers often introduce certain variables that are different, but the symbols chosen to represent these variables are often very similar.
For example, if one variable is denoted by $i$, the other will be $j$, instead of anything else, like $a, b,$ or $c$.
Likewise, if one variable is called $\mu$, the other will of course be called $\nu$.
Also, there is the $m, n$-pair from algebra books, which could get mixed up by some people. We could easily have defined matrices as having $r$ rows and $c$ columns, but why did mathematicians decide on $m$ and $n$?
Why have mathematicians chosen this style of notation?
Possibly too opinion-based, but I'll try to offer an answer.
Mathematicians often try to use similar letters for the same objects. So if you see the letters $\mu$ or $\nu$ in a measure theory textbook, chances are they will refer to measures. If you see $i$ or $j$, it will probably refer to integers. If you see $x,y,z$, it will more probably be real numbers, $z$ will be a complex number, and $f,g,h$ will be functions. Obviously, these are tendencies and not absolute rules.
Sure, you could name your measures $\mu$ and $x$, but would it really make it easier to read?