Why do SRs leave?
Several reason but the first question we can ask is (I don't have the answer in all cases): How long were those SRs in charge? 3 months, 3 years?
How hard was the pressure? Some do nothing others works 20 hours per day. Is that okay or can the workload change? Setting hours just like a doctor in the Netherlands (part time and never present).
Are these SRs missed? Not if there are 6 in one country because there are still left (is it necessary there are that many?).

We all will come to a stage that we leave, have better things to do, get ill or have the offline life asking for more attention. You will not find many who can generate a long term guaranteed income. I know I can't and never will unless I can move to a country where everything is for free.

No I don't believe the Steemit Team said an SC should give more attention to their own country fellows. If that would be the case I can upvote 8 Dutch people, I will adopt Belgium and South Africa and will ignore the rest.
All I heard (months ago) that it's not "allowed" to upvote in the communties you are a moderator at more than twice. I have no idea if this is true or more like made up or said to avoid that all votes go to the same community?
On the other hand SC teams give their guidelines and I can't recall it's ever said whatever they said is not allowed. The only SC I know literally said he will mainly focus/vote for his country was from Ukraine (also no one said: but hey you should look everywhere for that hidden gem). Well, we do but also focus on Europe since it's a fact most SC teams don't. If there's no SC from a European country (I don't count Ukraine because they rare vote for other European countries) this means most countries/communities we vote for will not receive a single vote (a bit has changed since I see that some SC follow me and vote for those on my list but I doubt they will if I am not a SC doubt as in I don't believe it).