Why am I not able to see the full text of a HathiTrust book?

From the HathiTrust copyright help page:

Many works in our collection are protected by copyright law, so we cannot ordinarily publicly display large portions of those protected works unless we have permission from the copyright holder. Where we have the right to show page images of works, we will make every effort to do so. We are currently displaying works that are in the public domain (such as US works published before 1923), uncopyrightable works (such as works of the US government), or works where we have permission from the copyright holder. If we cannot determine the copyright or permission status of a work, we restrict access to that work until we can establish its status.

Because of differences in international copyright laws, access is also restricted for users outside the United States to works published outside the United States after 1873. See Rights Status of Items for more detailed information about this restriction.