I think the main reason is that not many Ukrainians are terrorists, at least not unless you are Putin. The one's that seek shelter here will be vetted.
That's a bit different to horde's of young middle eastern men of conscription age coming to a country with an ideology and culture not to mention religion they despise.
There are countries nearer to them that share their culture and beliefs.
I haven't forgotten the terrorist attacks in London and Manchester by religious idiots.
I doubt Ukrainians wouldn't blow us up, they might even show a bit of gratitude for us having them and not set up their own "communities."