Why are emotional memories retained better than memories of humdrum experiences?

Because, it is call emotional memory.

Explanation:

In our minds we can learn, catch and take some moments that we live like an important thhing.

The emotion and memories works together since the information coding, to the recovery of some information.

What is the most important to remeber something is the relevance that it have in our life.

For example, maybe i don't remember what i lunch today, but i can remember when i had my title and my all familly was there.