Lulav was originally a Temple-only mitzva. Outside the Temple, it was taken only on the first day. This is confirmed by Plutarch, Josephus, and Philo — all describe the lulav as a Temple/priestly practice in the first century CE. The Pharisaic extension to one day in the medina was a deliberate policy of democratizing Temple mitzvos — making every Jew a participant in practices previously restricted to the Temple precinct. RYbZ's post-churban extension to seven days everywhere continued this democratization: the Temple's seven-day lulav becomes the whole nation's seven-day lulav, as a permanent memorial.