1) Depends on what you're using it for. Personally I think it tastes foul in salad dressing type applications. Olive oil is the gold standard for homemade salad dressings. I would imagine if you're frying donuts or potatoes or something, you'd want the more neutral flavor of canola instead of olive.
2) Rancidity issues lead to three problems. First is massively excessive processing which turns the healthy omega3 oils into trans fats, and also contamination from processing chemicals make the oil smell/taste weird compared to a more natural/real oil. The second is failure of the extensive processing means the stuff goes rancid really quickly, meaning you either dump half the bottle making it more expensive than olive or you eat rancid oil which is really unhealthy. The third problem is you can solve post-purchase rancidity issues by getting tiny little containers and using them up quickly, but that boosts the cost per oz way beyond something tastier, healthier, and longer shelf life, like olive oil or frankly pretty much anything else.
3) Its exceedingly heavily processed compared to most other off the shelf oils. This is not necessarily bad other than as noted above, but the more they screw around with it, the more chance to screw it up. Its easier to purchase un-screwed up evoo than canola.
4) Probably all that matters is your own local shopping experience, but I'm just saying where I live pretty much only shit grade canola is widely available, but pretty good olive (and other stuff) is available. I'm willing to believe there might exist a place, perhaps where you live, where really good excellent fresh properly minimally processed canola is available and no decent olive (and other) is available, although I find it pretty unlikely. The only oil worse than canola on my oil pecking order is generic "vegetable oil". Now that's not exactly made out of cucumbers carrots and tomatoes. Very disturbing, like a package at the butchers which refuses to officially identify itself beyond "meat, animal".
Those are the real world reasons why it usually sucks compared to olive oil. Outside of the real science based world, you run into stuff like over 90% of the canola is non-organic and GMO, so people who don't know anything about science are trained by advertising to respond by freaking out about it. I'm not so worried about pseudo religious "beliefs" as I am about real world biochemistry above. Everything is genetically modified to some extent or another, and as a class of product oil is processed a lot more than, say, lettuce, so being organic probably doesn't matter as much relatively for oil as for apples.
TLDR is its overly vulnerable to rancidity, so it either is super over processed to the point of icky, or has to be consumed while rancid which is icky, or goes bad fast so small bottles are more expensive per unit volume than a big jug of something that doesn't rot so quickly.
