Well I can guess at a possible strategy. The immediate effect is the Left convincing itself that Trump is afraid to do another debate after getting trounced by Biden. The secondary effect is that the expectations will be high for the third debate on Biden--the opposite of the expectations for first debate, and interest in watching it will be very high--which is ripe for Trump to move the needle on last debate before election day.
If there is no second debate do you really think there will be a third one?