It's a question of balance more than anything - overall the US does not wish to be seen as favouring one country more than others.
The reason for UK citizens not being able to participate in the green card lottery is that enough of us manage to immigrate by other legitimate means - so it can't be as tough as all that.
What is difficult is for unskilled brits to settle in the US - the quota (if you can call it that) is taken up by the skilled and those who have the right connections with US companies, leaving nothing for those without skills.
Whereas for somewhere like a central american country, which has few workers with scarce skills, the rest of the population do get a look in.