"Why did someone set up this magic training to use chants and hinder magical development?"
Easy answer: control. If over generations you make the populace think that all magic has to use chants whereas some group continues to exploit the knowledge of chantless magic casting, you still have a population that can use magic, but one where you still have a remarkable advantage. It's more commonly used with technology in science fiction (e.g. Stargate, where the government has tech millennia ahead of the non-classified stuff but lets the populace at large think the latest cutting edge tech is the best there is), but the same concept applies here.