In brief - the CRJ manual(s) don't give data (e.g. runway length required, etc.) for a takeoff in temperatures above 48°C (at 1100 feet elevation). Therefore it is not legal to do so, and the plane is grounded.
Why? The main reason is - Bombardier did not choose to calculate/provide information for higher temperatures.
Now, the deeper "why?" - why Bombardier chose to do (or not do) that, is not explained. The NP could not get a comment from Bombardier.
Thus Niallo's question.
The NP article gives some possible (speculative) explanations, but without an affirmative confirmation from Bombardier, they are just speculations. Maybe it is a limitation of some one individual component in the design - or maybe it was simply that Bombardier thought 48°C (50°C at sea level) was "enough," and didn't want to spend the extra time and money to extend the operating range calculations to (in their opinion) extremely unlikely temperatures. Or maybe it was something else.