If you don't know why there is country limit better understand it.
Employment based GC was started after long Family based GC was in place.
When Family based GC was created America created country limit to avoid few
country people gets GC based on chain immigration which is fair and no one
complains about it.
When they created Employment based GC they did just copy & paste work on
rules and regulation which includes country limit. It worked fine for
sometime but slowly started showing effects of inherited the family based GC
rules into Employment based GC.
If you are aware of different ways you can get employment based GC (H1, L1,
F1 etc) you know there is no check for country of origin. Country limit
would have been less issue if there is country limit in H1,L1,F1 etc.
Unfortunately that was not the case which was beneficial for all countries
expect few which holds majority of applicants.
When we don't DISCRIMINATE at the time of entry in to US, how would it makes
fair to DISCRIMINATE at the time of giving GC. Those people are already
here so they are not playing any role in increase or decrease of diversity (
the basic reason of having country limit).
Your statement " US Immigration policy is fair and wants to give people from
all countries a chance to comehere not just from highly populated countries
like India and China." is fully flawed because they are already here in US
and they don't prevent anyone from getting into US as there is no country
limit in H1,L1,F1 etc. As long as you can use any of those visas you can be
here in US legally and can apply for GC which has country limit (and will be
removed only for employment based GC if this bill gets passed).