Why is chkconfig no longer available in Ubuntu?

Best alternative IMHO is sysv-rc-conf To install just need to run the command:

sudo apt-get install sysv-rc-conf

Once installed run the command:

sudo sysv-rc-conf

You can check or uncheck the options to start a service on any level of execution and may even stop or start the services from this console. It is an indispensable tool to enable or disable applications on an permanently way to boot your ubuntu If you need a quick change, then you can use the CLI interface:

For example to stop ssh at levels 3 and 5 of execution:

sysv-rc-conf-off level 35 ssh

Atd to start in runlevels 2,3,4 and 5:

sysv-rc-conf on atd

If you want to know more:

man sysv-rc-conf