Makes me curious what state R was at the time, or whatever else could've been useful for deep learning, and the benefits of a new language vs adapting something that exists. Seems like it was a big investment
In my opinion R should thought of as an unbeatable graphical calculator, but an awful programming language.
R and its ecosystem have some unbeatable features, but, generally speaking, the "old", base R is too arcane to be widely useful. Also, being "made by statisticians for statisticians" should be a big warning sign.