If the point of downvoting were to help posters post better questions and answers, requiring downvoters to explain the thinking behind their downvotes might make sense. But in the Stack Exchange scheme, downvoting is primarily a tool for creating separation between perceived good/useful questions and answers and perceived bad/useless questions and answers.
One reason comments explaining downvotes are not required is that there is plenty of help available already.
How to write good answers
How to write good questions
Follow that guidance, particularly with regard to prior research and good documentary support of an answer, and you're less likely to be downvoted.
Welcome to EL&U. Requiring comments for downvotes is probably the single most commonly made suggestion among new users across Stack Exchange, and one that has consistently been rejected since the beginnings of Stack Overflow. See e.g.
Why isn't providing feedback mandatory on downvotes, and why are ideas suggesting such negatively received?