Why is my downstairs shower pressure lower than upstairs?
Your downstairs shower has a restriction of some sort in the water supply. Start at the showerhead and work backwards to find it. You need to check pressure while flow is happening - a restriction will allow full pressure when there's no flow, but will cause a large drop in pressure when there is flow.

For the easy possibilities, check any supply valves to be sure they are fully open, and check that you don't have a limed-up or otherwise overly restrictive showerhead. Other issues can be much more involved to find/resolve - my classic warhorse of a tale is the shower/tub with terrible flow on the hot water side, due (found after I reached the point of disassembling pipes, because something was clearly very wrong with them) to one elbow being 95% full of solder, leaving only a tiny hole for the water to pass through.