For some reason, C-152’s seem to hold a value a few rungs farther up the ladder, and are much more likely to be found in a more professional flight training environment. Much less unsupervised careless flying.
I suspect it has everything to do with the engine. The C152's engine has a 2400 hour TBO, the C150, 1800. That's an additional 600 hours flying. For a private pilot doing the average 50 or so hours a year, that's significant. For a flight school, it adds a couple of years between overhauls; so all things being equal, including engine times, the 152 will have a lot longer to go between overhauls.
Other factors that might relate to the accident rate. First, there are simply fewer 152s than 150s. Secondly, the lower value of the C150 means they are the most affordable aircraft out there, and many newly minted PPLs choose the 150 as their first aircraft (I know I did, back in 1982...and I miss the d-mned thing...not sexy, but cheap and fun to fly). I'm in Canada too...a good 150 with a few hundred hours remaining to TBO will fetch about $25-30,000 CDN. A similar 152 will cost around $40k. That's getting close to Piper Cherokee territory and frankly for most people a good PA28-140 will be a more capable aircraft than a C152 except for short-field performance.
And being newer, the professional flight schools will, as you note, prefer them to a 150; the smaller mom-and-pop shop type flight schools tend to go for the 150 as they are cheaper.
I don't think there's anything inherently unsafe about the 150. As a small two-seat trainer/personal aircraft, for a low-time pilot, it is probably much safer than, say, a Grumman AA1 or Traumahawk...