Its all about steering angle.
A front loader has to fit around exhaust pipes and front wheels.
Because of the exhaust, you need a little center distance on the booms, but then they (or the lift cylinders underneath them) will restrict the steering angle of a front wheel drive tractor with 24 or 28 inch front tires.
It also has to do with the breakout force it can generate, as you have to position the (long enough) lift cylinders in a favourable way.
Commercial loaders have the pivot point of the lift cylinder moved back a little from the mounting point of the boom, and have a straight boom: Backhoe loaders have very small front tires so they turn underneath the boom, articulated loaders dont have wheel steering angle at all because they turn the whole front end into the same direction as the wheels.
A straight boom allows for better positioning of the tipping cylinders, giving a better breakout force because they are less vertical. Also, as Tractortaylornut said, visibility of the bucket edges. Thats why with commercial backhoe loaders, or payloaders with parallel kinematics (instead of Z-bar kinematics), you'd see a straight boom.
On older farm loaders for 2wd tractors with 6.00-16 front tires, the curvature of the boom is just 10 to 15 degrees. Farm loader manufacturers later took the 70 to 80 degree booms as standard, to fit all possible tractors with different tire sizes.
About lift height, it doesnt make much difference, unless you move the angled loader boom so high that it hits the cab. You wouldnt want that lift height anyways, because you'd dump the muck over the tractor hood instead of in the spreader
