Poinsettias are associated with Christmas in the States. I don't know if they are a Christmas flower in other parts of the world or not and I didn't Google that this morning.
The first time I traveled to Costa Rica (my first time anywhere in Central America for that matter) I encountered the biggest poinsettias I'd ever seen They were trees and they were beautiful. But they messed with my head a little because those plants are generally only experienced where I'd lived my whole life in pots at Christmas. My grandma used to be able to keep any plant alive far longer than normal human beings and she's still have them around at Easter -- but most people quit watering them a few weeks after the tree comes down and they eventually end up in landfills.
I can tell you that living in Florida has changed that -- I take my Christmas poinsettias and repot them onto the patio for the winter to spring and , once they outgrow those pots, they go into the ground. I have a few of them in my garden. I even have a very small poinsettia tree in my garden that is deciding whether or not it wants to stay. I got it on deep clearance at Target last year.
So, the poinsettias on this nativity may be there as Christmas adornment, but the also make sense in a place where those flowers grow and bloom with great enthusiasm.