1 Culture clash
According to press, one of the main reasons Google is selling the business is that the engineers of Boston Dynamics didn't want to work with Google engineers after the acquisition.
2 No cash flow
Google said that the company is unlikely to produce any products that could be sold to people in the next few years, and so the company won't generate cash.
3 Failure to adapt to deep learning
According to one theory, Boston Dynamics' engineers could have found it hard to adapt to Google's deep learning approach when it comes to robotics, and as a result they didn't co-operate in the desired way.
4 Product development takes a lot of time
According to meeting minutes involving Boston Dynamics that were mistakenly released by Google to the public, the development of products for the company might take 10 years.
5 Fear of human replacement by robots
According to some internal mails at Google, the company was afraid that the reputation Boston Dynamics might get is that it's a company that creates robots that replace humans. (See Why do some people fear technology?)
6 It's hard to make robots do certain tasks
According to an article on Business Insider, it's still very hard to have a robot adapt to real changes in the environment, and as a result the company has a limited capacity to produce a useful product.
7 Google didn't want to be a defense contractor
According to one theory, Google could have wanted to sell the company to end all possible ties with the defense department as the company was initially funded by the defense department before Google bought it. (See How to identify a stock market bottom?)