That is the practice of the House. It is not a written rule. In a collegial system like the one in the House of Commons, a system that governs itself, it is best to use unwritten rules as often as possible. Written rules are less flexible and less practical. With written rules, any exceptions that might come up could often be problematic.
I cannot imagine that, in this day and age, we would find it acceptable that a Speaker be entirely unilingual. It might be acceptable that a Speaker is not perfectly bilingual. That is perhaps the case with the current Speaker, and that was the case with Mr. Milliken. Good intentions are very important. Here, there is no test like in the federal public service to determine whether an individual has the written and verbal understanding of the other official language. If there were a strict rule on linguistic capability, marginal cases would pose a problem.