I feel like it was a mix of not enough development time and purposely withholding features for future updates.
The original release date for ACNH was 2019 that was later pushed to 2020. So clearly they needed more development time than initially thought. I'm guessing they probably could have done with even more time even after the 2020 release date, but since it was already delayed once they were likely pushed to just release what they had by that time.
I also believe they had planned in adding new features and events in updates from the very beginning. We all know the game is meant to be played in real time despite the many people who time travel. To combat this issue locking holiday events behind updates make sense. In past games some players would play and time travel so much that within the first month of release or less they would have already experienced all the events and get burnt out on the game in no time. This new form of releasing new things in bits at a time help keep these excessive time travelling players coming back for the new stuff. They can still time travel all they want of course but the events won't be spoiled ahead of time. It also benefits non-time travelling players as we are less likely to have future events spoiled as in past game time travellers would often have event items early. It's not a perfect solution but I do like the attempt that was made to balance out those who time travel vs those who don't.
*Just to clarify (so I don't upset anyone) I'm fine with those who time travel even though I don't time travel myself. Play the game the way you want.
Overall I feel like many basic features were missing even in the original release of the game which would explain not enough development time. But most of the events were planned all along to be released in updates.
The original release date for ACNH was 2019 that was later pushed to 2020. So clearly they needed more development time than initially thought. I'm guessing they probably could have done with even more time even after the 2020 release date, but since it was already delayed once they were likely pushed to just release what they had by that time.
I also believe they had planned in adding new features and events in updates from the very beginning. We all know the game is meant to be played in real time despite the many people who time travel. To combat this issue locking holiday events behind updates make sense. In past games some players would play and time travel so much that within the first month of release or less they would have already experienced all the events and get burnt out on the game in no time. This new form of releasing new things in bits at a time help keep these excessive time travelling players coming back for the new stuff. They can still time travel all they want of course but the events won't be spoiled ahead of time. It also benefits non-time travelling players as we are less likely to have future events spoiled as in past game time travellers would often have event items early. It's not a perfect solution but I do like the attempt that was made to balance out those who time travel vs those who don't.
*Just to clarify (so I don't upset anyone) I'm fine with those who time travel even though I don't time travel myself. Play the game the way you want.
Overall I feel like many basic features were missing even in the original release of the game which would explain not enough development time. But most of the events were planned all along to be released in updates.