Are you talking about the French not using radios they already had, or are you talking about the French not buying and installing more radios? At that time two-way radios were a scarce and expensive technology, particularly the transmitters. The manufacturing capacity for tubes was limited and required very specialised manufacturing processes.
I've read translated Soviet documents from early WWII, and while they recognised that radios were a key technology for making the most effective use of tanks, the cost and the very limited manufacturing capacity for tubes meant that they could only outfit a small proportion of their tanks with them.
From what I have read about France in 1940, command and control communications was their Achilles heel at all levels. They were prepared to re-fight WWI with updated weapons, but were not prepared for the faster pace of war which the new weapons and logistical support allowed.