why #!/usr/bin/env ruby doesn't work in crontab?

If you installed ruby via rvm, ruby probably isn't in /usr/bin. Depending on where rvm is installed:

bash -c "source /usr/local/lib/rvm" && rails runner foo.bar

You probably added a source */rvm to your bashrc that is the correct rvm loading script.

your cron isn't inheriting your environment. try echoing "$PATH" to a file to see what it's set to.

You could also just do "PATH=/usr/bin/ruby && foo.rb"