The most common cause of poinsettia leaves falling off is due to over watering of the poinsettia plant.
When you water the poinsettia plant too much then the roots of the poinsettia plant gets root rot which then leads to the leaves falling off the poinsettia plant.
To prevent root rot of the poinsettia plant allow at least the top 50% of the soil to dry out before watering.
If you cut back on the watering then the poinsettia plants should keep their leaves without them falling off.
The poinsettia plants only need watering when the soil becomes dry to the touch and only need enough water to soak the soil.
Also allow the poinsettia plant enough sunlight because lack of sunlight can cause the leaves of the poinsettia plant to fall off.
It's best to place the poinsettia where it can get at least 6 hours of sunlight a day.