Lottery systems are generally considered fairer than first come first serve because they don't advantage people who apply early. Especially in an immigration-related process where circumstances are likely to dictate when you can apply, not everyone will have an opportunity to 'get in early'.
If you have a way to make the process simultaneously non-random, fair, and not have a cap (which I personally would probably be fine with eliminating but is presumably a non-starter for various political reasons), I'm sure some people would love to hear about it.
Is that really the purpose of the family-reunification program?
Genuinely random selection averts loads of really nasty problems in a system where you need to pick things even though humans tend to be sure some other approach would work better.
On an individual basis of course not, but you don't think in the aggregate better candidates can be chosen?