top tube angle on steel and carbon road bikes You generally see the top tube on steel bikes being parallel to the ground, whereas on a lot of carbon bikes it's angled. Why is that?

(my observations are obviously limited, but it seems like a trend)

Pictures for comparison:
The only real advantage to a compact geometry road bike is for the manufacturers. Since standover- which is the distance from the top tube to your crotch when you straddle the bike with your feet on the ground- is increased, one size of frame will fit a wider height range of people and therefore the manufacturer does not have to make as many different frame sizes. This is valuable for aluminum and steel bike frames, but pays off the most in the world of monocoque carbon frames as each size requires its own mold(s) and for higher end bikes, its own layup schedule. Final fitting is done with saddle height &amp; fore/aft position along with steerer stack height, stem length &amp; angle. Ultimately there is no difference in fit between compact and classic road bike geometry.

Compact geometry frames use ever so slightly less material in the frame, but primarily they are just the new aesthetic of road bikes as compared to those of the 1980s era and before, and that era of bike was typically steel.