Why is R such a popular data science tool?

The most important reason I think is that it is free and open sourced. Universities usually used to go for the more mature statistical tools SPSS or STATA - student package or a cheaper version of some kind. Students would pay up a small price and so would the universities. But their preference shifted to R as it started becoming much more powerful in its own way, and flexible in comparison to the other 3.
Now, by all means, toolkits like SAS are the market leaders when it comes to analysis. But think about it. Employers that learnt Data Science through R are inclined to think that if someone doesn't know R, they aren't really in it. It is sadly true in most cases. So your hiring chances drop too. R definitely has a steep learning curve. Hence in the beginning years, you are bound to lose sight of doing the actual data analysis and are more focused on getting the syntax right, or getting the code to work, which is kind of a bad thing. Most large scale companies, where cost doesn't matter much, stick to the other 3. That being said, R is still much more flexible and a programming language that gives it an upper hand in some ways. It's not just a statistical tool. It's a language.
My knowledge on this is actually very limited, and this answer is based on an article I read, and I don't remember which though. My apologies.