Because angle/side eject on a lever action is an unholy abomination, and putting a scope on a lever action is a violation of the laws of nature. If John Moses Browning wanted us to have scopes on our lever guns, he would have designed a bolt action.
Now go do 12 hail mary's, 10 our father's, and flog yourself 10 times for this sacrilegious heresy, and never have these Satanic thoughts again.
=)
Okay now that the fun is over...
About the only reason for side eject is, as you surmise, the use of a scope. Problem is, putting a scope on a lever gun pretty much eliminates all of the best qualities for which people admire lever guns. A stock Winchester (because that is the ONLY lever gun) is a rugged, dependable, naturally pointing, quick to shoulder-aim-fire, fast follow up shot, type of gun. It is easily carried in a scabbard, slung on the back, tied to a canoe (my preference), hung on a 4 wheeler or snow machine, behind the seat of the truck, etc etc etc.
The moment you put a scope on a lever gun, ALL of that is taken away. Then it becomes this slow to aim and fire, delicate, thing that you have to keep in a padded case until you're ready to shoot.
Now I took a hog last year with a friends 30-30 Marlin with a scope. It worked well. Even at 25 yards, the scope let me be certain for that behind the ear shot hogs require, which would have been more challenging without the scope. I missed the second hog, one the run, clean because of a lack of target acquisition.
Once you put a scope on a lever gun, it becomes a single shot affair. Given the inherent diminished accuracy of the flat nose bullets required in a lever gun, compared to pointy bullets in a bolt gun, there is no reason to give up that accuracy unless you get all the benefits that come with a lever action.