I don't understand why Tango has to be an integrated built-in solution rather than an accessory that could be added to any Android phone.
Because hardware accessories still require standardized hardware to work with (or complex adaptors, etc.), and are more cumbersome, which limits their appeal. Tango requires specialized hardware, and it makes a lot more sense to integrate it than to make it an accessory.
0: http://www.flir.com/flirone/ios-android/ 1: http://www.thermal.com/products/compactxr 2: http://gizmodo.com/caterpillars-new-s60-is-the-first-smartph...
Then maybe you'll see vendors offering a combination of tango and base-level frames? Just switch between them as required.