Why can't I drive this Tapcon into concrete?
Aside from not using hammer mode, as suggested in the comments, I will report that I have had HIGHLY variable results with Tapcon's in old poured concrete - I don't recall what they claim to work in, but I begin to suspect that concrete block or green (not yet fully cured) poured concrete is more their cup of tea, or perhaps "concrete made only with "soft" rocks for aggregate."  Some work as they should, some stick partway in, some go all the way in and don't hold; I think it's luck of the draw on what the concrete right at that particular hole consists of. Sometimes the threads shear right off (bit of hard aggregate they can't cut into?) 

You can switch to driving them with a ratchet, but you may find that the threads shear off, or the entire shaft shears.

Since this is a vertical-down hole, you might try vacuuming it out before driving the Tapcon.

I've gone back to traditional anchors. Our local aggregate tends towards hard rocks, not crushed limestone.

They work OK as big blue overpriced wood screws ;^) which will let me use up the boxes.