The problem with sponsored characters is that it's a drain of time on the part of whichever immortal is looking over the apps.
I know Krrx goes through and checks the pinfos on each character the person has played, as well as notes on the account. If there's a large number of characters on there, that's a substantial chunk. When a sponsored app is rejected (and I will, perhaps cynically, observe that often the people applying for sponsored characters are not the ones one would be overjoyed at the thought of their running a whatever), he writes a fairly detailed letter as to why they're being rejected.
A lot of times people are not scaling their requests very well. If someone has no karma at all, they're better off asking for something like a desert elf or vivaduan (relatively low on the karma scale) than something substantially higher up on the food chain, such asa psionicist or mindbender.
The karma system is simply a way of saving time. Someone having enough karma to make a mul means that the staff trusts that person to a) roleplay a mul well, b) not to abuse the abilities of that role and c) to act in a way that benefits the game, both ic and ooc.
And let me point out as well, that the reasons for not having karma may be varied, including, but not limited to: not having played long enough for the staff to have much opinion as far as quality of play goes; having powergamed and abused bugs; having roleplayed poorly; having done something that does not benefit the game, such as spreading ooc information or attempting to hack the machine, etc.