Because Algernon is missing out on the cognitive progress he has developed and the lab team doesn't want Charlie to believe it will happen to him either.
Explanation:
Algernon is the mouse that served as a guinea pig for the surgical procedure that allowed Charlie to overcome his mental disability. Like Charlie, Algernou greatly improved his cognitive ability, his independence and his ability to solve problems. However, this does not seem to be lasting, as Algernou is losing those skills and returning to his state of mental disability. For this reason, Charlie is not invited to see him, as he may believe that this will also happen to him.
