Which is why or that is why?
We live in a data and information-rich society, which is why it is advantageous to look at every opportunity to increase one's knowledge base and set skills.

Shouldn't it be 'that is why'? How to decide?

Question

Which is why or that is why?
We live in a data and information-rich society, which is why it is advantageous to look at every opportunity to increase one's knowledge base and set skills.

Shouldn't it be 'that is why'? How to decide?

Answer

It can work either way. In my opinion, the first option is good for connecting the two ideas, whereas the second option creates some empty “space” for reader to digest the information on their own:

Use “which” (a relative pronoun) after a comma to introduce a non-restrictive (non-defining) relative clause.

Use “that” (a demonstrative pronoun) after a period (full stop) to start an entirely new sentence.

Answer

I would use 'that is why' with a new sentence, rather than after a comma.