CGP Grey has a great video examining different boarding methods and why some are slower or faster than others: https://www.youtube.com/watch?v=oAHbLRjF0vo Also noting that the way we deplane (front to back) is the slowest method, heh.
1) Business and 1st class make a lot of money for airlines. Not letting them board first (although personally I've always wondered why this is desirable) would devalue their tickets and make less profit for the airline.
2) Airplanes have to undergo certain maintenance checks or procedures whenever they land. Those take some fixed amount of time. There is no incentive to speed up the boarding/deplaning process past that fixed amount of time, since it won't be the limiting factor.