You can't overload a team with all the options at once. You'll be more successful if you can run, say 40%, well rather than 80+% sort of well. Learning to run an offense is about repetition, and repetition takes time. When implementing a new offense, you simply just don't have enough time to get all the repetition needed to roll out the entire offense. So, the first issue, as it relates to the whole package, is a time insufficiency issue.

The second issue you asked about is about new players coming in to the program each year. They too will have a learning curve learning the new system. This is why the best programs have young players, typically, spending their frosh and soph years learning the system. This is where they get repetition time to learn the offense. Now, you always have outlier kids who come in and can pick up things quickly and can insert themselves into the first teams early, but it's not as common. Even for these special kids, the offense is often dumbed down in its complexity while they learn week by week.