The following question, Diminutive forms in English , has received a number of comments by experienced and, I think, authoritative users that appear to contradict and somehow reject what I am asking. It seems that it is an elementary issue that I simply fail to see. Despite a very good tentative answer I still don't understand if the issue I am trying to raise is recognised or not. Could ELL, given the basic question, be more helpful in that respect?
Why a language is as it is questions are almost never constructive. The answer is inevitably that a bunch of effectively random historical forces combined to produce the language we have now. This doesn't mean that those historical forces can't be explained, but to properly answer "Why are diminutives in English not always formed by adding a suffix, like in French for instance?" an answer would need to cover the individual fate of several unrelated affixes, which each have an idiosyncratic history unrelated to the others.
But it's extremely rare in any language for there ever to be only one way to do something. Why are diminutives not always formed by adding a suffix? Because language is never so rigid. If you want to try taking the first question to Linguistics.SE, I'd suggest phrasing it in terms of why it's less common in English than in other languages (and to improve such a question, concrete data on languages where it is more common would be good.)
Your second question is reasonable, but should be asked by itself. It belongs on ELU not ELL. But it's a very different question, and asking the two of them makes it too broad.