Because it would have been announced like all other program changes that are planned for the future. The Liberals have no plans to add a lottery. If they did, we would have heard about it by now.
The closest thing to a "lottery" that has been added is that for parent/grandparent sponsorship, it's no longer first in, first out - but random selection. However this only applies to Canadian PRs and citizens who are sponsoring their parents/grandparents for PR. It's not a general lottery like the US has.