They had enough
Even with all of the success (and money) of their new show, it was painfully obvious that being in such close proximity at all times of the day wasn’t doing the couple any good. Their marital problems leaked onto the set, so following the wrap-up of shooting their final episode, the couple decided that enough was enough.
It was reported that Desi felt a wedge was driving them apart. The stresses of running a production company and being the sidekick to his wife were causing him to lose sleep. But the thing that broke his confidence and drove him to substance abuse was when his daughter, Lucie, called him “Mr. Ball.”