Evolution of Definite Articles in Indo-European Languages

You're forgetting Slavic languages which generally don't have definite or indefinite articles (with some borderline exceptions) and Lithuanian. You can see the spread of definiteness marking in the WALS map.

The etymological origins of definiteness marking are quite easy to identify.

Definite articles generally originate in some sort of demonstrative pronoun. And indefinite articles often come from words like 'one' or 'some'. It's also not difficult to imagine why they might be found useful by speakers. In fact, in modern spoken Czech you can sort of see incipient use of demonstratives to mark definiteness in some situations.

But that doesn't answer the question of why definiteness marking has developed in some languages and not others.

Here the most likely answer is areal diffusion.

While it is likely that definiteness marking in disparate languages has evolved independently, it is also likely that the clusters of use of articles are due to areal influences.

A good example of this is the Balkan language area (the famous Sprachbund) where most of its members (except Greek) have a postponed definite article which includes the Slavic members who could not have inherited definiteness historically and the one Romance language where definiteness looks nothing like in the other languages in the family.