The reason is that the choke point benefits these companies. If you cleared the backlog so that a green card was a 12 month process, the companies would quit asking for H1-B's unless they really wanted them.

If US gives X green cards to nationals of Y countries (X>>Y) in a year, each country is allocated X/Y number of green cards.

This puts Indians, Chinese and other high population countries at a disadvantage.

An Indian applying for green card in EB-3 category (minimum qualifications being undergrad degree + 3 yrs experience i think) has to wait for ~12 years from the date his gc process was started. EB-2 (advanced degree and/or 5+ yrs experience) is ~10 years (these are my ballpark numbers)

Its not the turnaround time, but the concept of diversity based green cards that slows down the process.