Why Yamnaya people were Dark but Andronovo people were light eyed and haired ?

[Daugakrit]

Because Yamnaya precedes Andronovo by a couple thousands of years, and only during later times did the blond-mania mutation become prevalent through sexual selection (it's purely cosmetic and there is no performance benefit).

People with blue eyes see better in the dark. I thought this was common knowledge.

[War Chef]

People with blue eyes see better in the dark. I thought this was common knowledge.

Even if it were the case, it doesn't make sense that this would confer a benefit statistically significant enough to make light-hair become prevelent in a short period of time.

It is well known that people with lighter eyes tend to be more sensitive to light, a result of having less pigment in the iris to protect them from sunlight. That can place them at a greater risk of macular degeneration and other eye-related problems. But whether that extends to vision is not clear.

If there are any differences, they seem to be subtle. There is little or no evidence that darker eye color means greater visual acuity, but one theory holds that it does produce better reaction times.

Studies have examined this by looking at sports performance. One, at the University of Louisville, found that dark-eyed people performed better at “reactive-type tasks,” like hitting balls, playing defense in a football game and boxing. But people with light eyes did better at “self-paced tasks,” like hitting a golf ball, throwing baseballs or bowling. A similar study of college students found that subjects with darker eyes performed better at hitting racquetballs.

But other studies have challenged those findings, including one that looked at rugby players. Scientists say more study is needed.

https://www.nytimes.com/2009/01/20/health/20real.html

[Daugakrit]

Even if it were the case, it doesn't make sense that this would confer a benefit statistically significant enough to make light-hair become prevelent in a short period of time.
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The "quickly evolved sexual selection" hypothesis is just one of a few, though. There are a lot of explanations as to why lighter skin and hair might be beneficial.
I've read somewhere that some Neanderthals also had blonde hair and blue eyes as well. Not sure about the validity of that, might research it later.