3. Forwards not contributing enough defensively
The Chelsea forwards have not contributed enough defensively. They usually do not press the opposition defenders much, and are not used to falling back to the middle third either.
As a result, Chelsea have often lacked in numbers in the middle third, and their opponents have taken advantage of that. Both Higuain and Pedro are on the wrong side of 30, and not very comfortable dropping down very frequently.
Moreover, Eden Hazard prefers to stay in the wing and cut in from there, but avoids his defensive duties sometimes. Also, Chelsea's front line concedes possession too easily at times, and do not press immediately after that. That is in stark contrast with a team like Barcelona, whose forwards press very high to win the ball back.
Barcelona have also been playing a 4-3-3 for more than a decade now, but their high-press allows them to retain the ball more frequently in the final third.