The deb packages depend on python-support, which no longer exists in Ubuntu 16.04 (xenial).
$ sudo apt install cassandra
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
cassandra : Depends: python-support (>= 0.90.0) but it is not installable
Recommends: ntp but it is not going to be installed or
E: Unable to correct problems, you have held broken packages.