The fact that Tango isn't on the Pixel means Google has no confidence in it and nobody should invest in it as Google will likely abandon it quickly. It's a shame, I think it's great idea, and they'll only end up playing catch up later if they do.
1. The technology is still new and not yet polished
2. Pixel 1 was a rushed job made in 9 months
3. Tango needs extra hardware that increases cost
I have a feeling that Pixel 2 might have it though, as they had much more time working on it and the technology itself has matured a lot more since, as we saw at IO. Not only that, they also showed off VPS which is a visual position system for indoors using Tango. Definitely a killer feature for the Pixel.
But it's definitely on fewer total devices, so Apple has a real edge here.