[madaxe_again]

Because you have to haul up a great big airframe, and you need a landing strip, and you limit your configuration.

With this variety of thing you can strap multiple first-stage rockets together, blast a great big payload into space, and then the first stages return safely for rapid re-use - while your payload stays up.

The space shuttle had a pretty small payload bay, and only came in the one configuration - no flexibility, like this provides.

Also, with a passively slowed vessel, like a shuttle, you have a huge amount of heat to dissipate, which requires thermal tiles (yet more mass and refit each launch), whereas this slows in several burns throughout re-entry, meaning that the thermal stresses are nothing like you have on a shuttle re-entry.