Apologies if this sounds like an ignorant question. Why isn't this common knowledge? How hard is it to convince people that sugar is bad?
And usually (micro)nutriments have trade-offs : good for somethings bad for others.
Look at the different kind of diet, they are all based on different researches and can be extremely different while all are using 'valid' research data. The main idea behind them is usually to have 'good' proportions between macronutrients (each diet define a different proportion) while keeping the number of kcal in the range you want to reach your objectives.
For a first rule of thumb that I would use: abusing in any direction is bad. Sugar is ok in 'small' quantities, fat, etc also. However reality is way more messy :)
I did not believe sugar was the problem until randomly decided to have a try while I was trying to lose some weight. Tried to eat less walk/run around 8km per day for around a year, it helped a little. I did manage to lose more or less 10kg. Unfortunately, problems with the project and had to work over time and thus stopped running, eat lots of junk food for roughly a month, during which time drunk lots of Coke and when it was over, I found my self picked up 15kg. Then I decided to try something new, stopped drinking Coke and eat less carbs in diets by eating more meat and vegs. And the result was much better than I thought: I lost almost 20kg in less than 3 months and the exercise I took actually dropped a little bit.
140 calories for a can of Coke is pretty modest; I just ate a third of a lemon meringue containing 500 calories - most supermarket food is calorific muck, cheap to manufacture yet tasty enough.
Tests on athletes (some now decades old) show even moderate sucrose consumption reduces athletic performance and health.
For everything the key is to be reasonable with quantity.
It's fine to enjoy Coca-Cola. It's best to be reasonable with quantity.
It's kind of with everything else... people are addicted (I certainly still am, after decades of sugar abuse). Then there's a whole culture around that addiction and our natural craving for easy calories (fuck evolution, really).
As with all of those issues (religion, ideology, conspiracy theories) you need to "nudge" people in the right direction so they can "discover" the issues themselves. Facts don't help nearly as much as one might hope and a lot of it is correlation obfuscated by wishful thinking and genetic variability ("but my cousin Joe always drinks a coke a day and still isn't fat!").
Ever tried to convice a smoker of non-smoking? An anti-vaxxer of his narccisistic ignorance? It's always the same pattern.