[YoumuChan]

Economy of scale: starship can be "mass"-produced

Material: stainless steel is much cheaper

Percentage of reusability: boosters of shuttle cannot be reused, maintenance of shuttle itself is also very expensive (heat shields were pricey). whereas the starship stack has higher reuse percentage and allegedly cheaper to maintain.

[moffkalast]

The shuttle was not even reusable by any modern metric, the main tank was always expended, the boosters had to be recovered, fully disassembled and cleaned.

[pfdietz]

I'm not even sure the SRM case segments could be easily reused, given the tremendous stress. They were made of a very high strength steel (maraging steel, with a yield stress of something like 250,000 psi) operated with a safety factor of 1.4.