It’s a lot easier for the parser and you this way. contains seems unambiguous, but without the method call’s mandatory parentheses, you have to specify its precedence in the massive hierachy of operators so something like "pizza" contains "iz" * "za" works. Then you have to communicate an unfamiliar operator and its precedence with everyone you share your code with.
Operator suffixing preserves familiarity. ∈ᵒᶜᶜᵘʳˢⁱⁿ looks like an ∈ and inherits its characteristics like precedence and associativity. in is not exactly like ∈ in this way, though, can’t suffix it to make a derived operator. Can’t dot-broadcast it, either.