Conventional smoke detectors use radioactive isotopes (americium). I imagine Nest wanted to avoid putting radioactive materials in their shiny consumer products.
Some do. Many others also use the photoelectric smoke sensor that Protect does, and some use both. The photoelectric type is better at detecting smoldering fires. The ionization (radioactive) type is better for detecting flames. Perhaps with a temperature sensor in the Protect, they thought they were covering their bases.
That's a shame, since the only place a Nest Protect apparently belongs is a nuclear waste storage facility if that's what it takes to get them to shut up after a false alarm.