For every 1 inch of rise, the ramp must extend out 12".
Every 20 feet of ramp must have a landing. Consider this: landings are required, by code, in stairways for people to rest. Landings are required, by code, in ramps for the same reason - to rest. Every 20 feet of ramp to a person in a wheelchair is the equivalent of one flight of stairs.
Ramps take a lot of space. A ramp to go up 20 inches would require 20 feet (at approximately 3 feet wide, this means 60 square feet of ramp).
Ramps are difficult to use. They require strength, coordination, and skilled navigation, often for an extended period of time.
Ramps are not inexepensive. By the time you have a ramp to lift 30-40", you could be paying for a Vertical Platform lift such as the Opal.
In Public Buildings ramps are not seen by disabled persons as true accessibility.