Everyday Astronaut explained this pretty well: https://www.youtube.com/watch?v=SCCw_M8MAU0&t=110s
The Shuttle's heatshield had to deflect significantly more energy because of the different landing approach.
The Shuttle's heatshield had to deflect significantly more energy because of the different landing approach.
Unfortunately, part of the design requirements was to return a large amount of mass from orbit which means it needed to be designed to dump a rather obscene amount of energy into the upper atmosphere. This meant it could not do a rapid decent and because it would overheat, and thus needed to stay in the upper atmosphere for a long time. Which meant it's thermal tiles needed to operate for much longer time periods requiring extreme amounts of thermal protection.
PS: The wings also added even more weight issues.