Few houses, even offices in Brazil, have a central air conditioner / air conditioner, so we hardly see products like thermostats being marketed here. But in the USA this is quite common and one of the big names Nest Thermostat, from the homonymous company founded by Tony Fadell (former Apple executive) and acquired by Google.
In mid 2012, Apple stores (physical and online) started selling the product, but now both Nest Thermostat and Nest Protect are gone from Ma's (real and virtual) shelves, as reported by Mashable.
The reason? The first batch of devices compatible with HomeKit has already been launched (such as the ecobee3 thermostat) and Apple wants to honor, highlight its break partners, it gives another pin on Google.
Some may think that Nest products could at some point become compatible with HomeKit, but the likelihood of that happening now is little since Google also has its own project to connect devices, called Brillo. It is quite possible that Nest has a central role in this project, which most likely takes HomeKit out of the picture.
We know that Apple is reducing the number of accessories sold in its stores, so filing these products as it already did with the Fitbit line also makes no sense.