Because Apple does not need Nest at this time, The same way Apple does not need NetFlix and/or Hulu and/or Pandora.
Why did Google buy Nest ?
Google does not need Nest either at this time but had invested in Nest and have an interest is keeping their investment and friend's business alive.
Without Google, Honeywell would kill Nest with legal fees.
From what I heard, they were able to best Honeywell in the courts. How could they have gotten 3 BILLION dollars if they were desperate to be saved? You have to have some pretty awesome leverage to get 3.. BILLION.. Dollars. They were not a struggling company.