They are popular because ARs are (were) Restricted, and the AK prohibited.
If we had access to $500 AR15s and AK47s like they do down south, the SKS wouldn't be nearly as popular. Instead, In Canada you are looking at $1000+ for a mediocre gun that probably won't be any more accurate or reliable. Plus for a lot of people starting in the gun world, its accessible - I didn't have much money when I got into guns (still dont really) and it was something that I could buy a gun, buy a spam can of ammo, and go put a bunch of rounds down range without a big cost incurred.
Thus, the appeal of a $200-300 gun with cheaply available surplus ammo. Of course, ammo prices aren't what they used to be, and the availability of NR Semis is much better than it was 5 years ago (or it was until the Liberal OiC and subsequent FRT changes the Horse Cops have been doing), so you can also see why their popularity may have declined in the last few years.