Because most people don't drive at night.
If a country has abundant solar power (probably not Finland!) it would make more sense to wire up workplace parking lots so that EVs could absorb the excess midday electricity.
That would be nighttime.
Some EVs can automatically charge at these times, and sometimes you get discounts for charging at these times, but you can also usually just set a timer on your charger.
https://www.oeb.ca/consumer-information-and-protection/elect...
For example the above suggests it's usually nighttime, but weekends and statutory holidays are also good.
As an aside, I wonder how much money an economy could save by having a new "cheap electricity holiday' which chooses one or two days a year that they announce a month in advance as national holidays when an extreme weather event is predicted.
We have electricity run to almost everywhere there is a road, and every building that we spend any significant amount of time in. We can charge at work or on the street during the day. Basically anywhere a car is stationary.
With the right incentives and infrastructure we can shift electric vehicle charging to daytime, or whenever there is excess generating ability coming from the grid.