I think that the "think only positive" attitude is/was popular because usually, people find it hard to think positive.
In my opinion, they are more likely to sink into negative thoughts than in positive ones.

but I agree with you that one shouldn't avoid the negative thoughts.
negative thoughts like self-criticism can teach you very useful lessons in life.

to me combining negative and positive thinking is like when you fall you should remember what was the cause to your falling to avoid falling the next time (negative thinking) and getting up quickly, ignoring the pain as fast as you can help you to feel better faster (positive thinking).