Because it's harder to work with. Porcelain is dense and non-porous, which is exactly why it's so durable — but it means it won't bond with standard mortar. It needs a slurry primer on the back of every slab and a proper porcelain-grade adhesive, and cutting it needs the right blades. The material cost is only part of the picture. A cheap porcelain installation is usually one where the priming has been skipped, and those slabs come loose.
External-grade porcelain is manufactured with a textured surface and rated for outdoor slip resistance — it's typically better in the wet than a smooth honed natural stone. What you must not do is use indoor porcelain tiles outside. They look similar in a showroom and they are not the same product. We only lay external-grade slabs.
Largely, no. Porcelain is non-porous, so it doesn't absorb wine, grease or leaf tannins the way sandstone can, and it doesn't need sealing. It's frost-resistant and it won't fade. Maintenance is essentially washing it. That low-maintenance quality is what people are really paying the premium for.
