It’s a combination of factors that make fast descents on a road bike unnerving.
-Geometry – the bike is designed to be efficient for chopping along on the flat and climbing, but even “relaxed” road bike geometry can feel twitchy pointed down at high speed.
-Tyres – again that thin highly pressurized strip of rubber is intended to be most efficient in nominal conditions. Really high corner speeds especially with adverse camber, moisture, grit or dirt on the surface can be extremely sketchy.
-Braking – It’s not that road brakes are “Bad” it’s that you need to read ahead and use them early and taken off enough speed to account for the acceleration you get through the corners where you really can’t get away with touching them…
-Environment – we have already mentioned some of the things that can get on the surface of the road and affect grip but then there’s the other aspects of the environment around you, pot holes come up faster and you and the bike feel them. And you’ll be conscious of what you are liable to hit should you run out of road; kerbs, walls, fences, trees, houses and various items of road furniture are not very nice ways to abruptly decelerate a human being.
All of these things coupled with the prospect of skin loss should you “just” tumble and slide will play on your mind and make the road suddenly feel quite narrow.
Road bikes don’t descend like MTBs less rolling resistance means they accelerate under gravity far faster and will get to much higher speeds far quicker not really being designed for this sort of operation means that a road bike feels unnerved and scary to use at 50mph+…
Best advice is to read the road ahead, act carefully and deliberately, use as much road as you can and use your brakes early in a straight line not through those 35º corners that now feel like deadly 90º bends and stay as relaxed as you can, tension will not help