Earlier, if we registered on the Ubuntu site, we would get a CD shipped freely to our home. Why has Ubuntu stopped it?
Ubuntu used to have a "ShipIt" service which shipped free CDs.
This has now closed.
They have put up a notice on the former ShipIt website here:
You can read their official blog post about their decision, made in April 2011, here:
Here's an excerpt:
They go on to talk about a few ways that you may still be able to get a free Ubuntu CD.
TL;DR: They spent huge amounts of money shipping free CDs and decided that no longer makes sense, citing not much more than "technology moves on".
Looks like administration and cost were cited as the reason for stopping it (along with better internet connection speeds available now). You can still purchase one from the Ubuntu store, and can now try Ubuntu out online at http://ubuntu.com/tour