Like foreignmusic said, the problem with concrete with nothing over it but straw is the horses legs will become sore.
Think about how you feel after you stand on something hard all day.
That is why cashiers at Walmat have those rubber mats to stand on, so their legs do not get fatigued.
The other problem with concrete is that it would take LOTS of straw to make it such that the horse would not go thru it when laying down and standing up. This will cause sores and maybe an injury when they slip on the wet concrete.
Concrete is also impervious. Which is fine if you have bedding that absorbs all the urine (saw dust, shavings, etc). But straw does not absorb that well.