Apparently he has had symptoms for years. Maybe the condition had progressed over the years to the point where studios just couldn't compensate for it any more. .The following is from an article on the DailyMail website:
The world was shocked by the news this week that Willis, 67, is retiring from acting after he was diagnosed with aphasia, a brain condition that affects his ability to understand language.
Bruce has reportedly been struggling with cognitive issues on the sets of his films for years - and even needed an earpiece to feed him lines - long before his family announced on Wednesday that the famed actor had been diagnosed with a brain condition.
An unnamed source told Page Six his declining cognitive ability had been an open secret in Hollywood as the actor repeatedly had trouble acting in his films.
According to the National Aphasia Association, the condition is an impairment of language, affecting the production or comprehension of speech and the ability to read or write.
The association reports the condition is always due to injury to the brain, most commonly from a stroke, particularly in older individuals.
Head trauma, brain tumors or from infections can also cause aphasia.
The only treatment is speech and language therapies, with involvement from family members - but even with that, the condition could worsen.
It is currently unclear what lead to Willis' diagnosis.