Why is socialism still so popular?

Why would the NDP want to keep socialism in its charter?

Because a lot of voters still think socialism is a good idea. Socialism's defining myth is voters can get something for nothing, all they have to do is vote for the NDP.  The NDP left out the part about how everything we consume has to be produced. Just because voters want something doesn't mean they have the right to just take it away from the producer. It also doesn't mean this taking is can go on forever.

Those plundered aren't too happy about it, and they react. When what is left to take starts to fall but what voters want to take continues to rise, voters, who were told that votes somehow create things to take, aren't too happy either. The politicians get worried about losing the next election and are then forced to ration. Rationing is unpopular but the usual scapegoats, such as greedy businessman, Arab oil sheikhs, the rich or bad weather will be dragged out to justified it. It's never the government's fault.

When rationing doesn't work anymore, those who are still able to produce must be motivated to produce more for even less. Eventually, they must be forced to produce -- they are enslaved.

How can socialism lead to the collapse of a nation?

Voting produces nothing.
Force produces less and less.