When you let the people vote, their vote should be final. Otherwise, you are able to arbitrarily ignore election results when the outcome doesn't suit your arbitrary rules.
The idea of enforcing diversity after an election can in some cases lead to nullifying the election altogether.
There is a real life example here. Although it was never put into action, there was a suggestion for the Belgian elected officials (Senate) to be 50% male, 50% female. It sounds good, right?
Except that there was such a discrepancy between the amounts of male and female candidates, that when you did the math, you realized that every female candidate would be guaranteed a seat. That is not acceptable in a democracy.
As a simple example, let's say that 20% of your population, and therefore 20% of your candidates, have Elbonian heritage and are considered a minority.
You have 5 open seats. When applying your diversity rules, to reflect the populace's diversity in the group of elected officials, this means that you'd expect 1 elected member to be Elbonian.
As it turns out, while this is pure coincidence, all Elbonian candidates are favored by the populace. Therefore, all 5 seats are assigned to Elbonian officials.
Do you feel the need to interject a diversity rule here? That's a very dangerous precendent to set. You've effectively removed 4 out of 5 elected officials.
Alternatively, again just pure coincidence, there is only one Elbonian out of hundreds of candidates, and this candidate is not actually a good choice compared to the other candidates. Five non-Elbonians are elected to office.
Do you feel the need to interject a diversity rule here?
If you intervene and ensure that this single Elbonian candidate is given a seat, then you've effectively ensured that this candidate was always going to get a seat and therefore did not need to be elected in the first place.
Your nation has 50% religious native people, and 50% non-religious. As it turns out, due to their cultural heritage, Elbonians are almost fully all (99%) religious.
In the election, the Elbonian candidates are all non-religious (from that 1%). Do they reflect the population? If you say no, then you are effectively excluding them from being elected because of other people's life choices.
Let's say both your native populace and the Elbonians are 50% religious. The same is true of your candidates. However, as it turns out, purely coincidentally, you end up with 5 religious native elected candidates.
If you enforce diversity, how do you do it? Maybe you pick 2 religious native people and 2 non-religious, but then you've already put two people in office who were not elected.
So let's say you keep 4 elected (coincidentally religious) native officials in their seats, but you change the 5th seat to an Elbonian candidate. Let's say there are two Elbonian candidates. The one with the most votes is religious. The one in second place is non-religious.
If you want religious diversity, you'd pick the second candidate, thereby subverting the election results. If you pick the candidate with the most votes, then you end up violating your diversity rule for religious diversity.
Let's say you abandon the idea of post-election changes, and instead simply ensure that the pre-election candidate list reflects the diversity of your populace.
This sounds much better, no? Except that it completely misses the point of ensuring diversity.
Let's say there are 50 candidates allowed. 20 native religious candidates, 20 native non-religious candidates, 5 Elbonian religious candidates, 5 Elbonian non-religious candidates.
However, all candidates are left-wing, as your nation is predominantly predominantly left-wing. A right-wing candidate presents himself, who is Elbonian and non-religious.
What are you going to do?
- Disallow him to enter unless he finds candidates to maintain the diversity? That wouldn't be fair. A candidate's electability does not hinge on whether other candidates with contrary opinions can be found.
- Put him on the candidate list? Now your balance is upset. While one Elbonian candidate doesn't ruin the balance, 10 new Elbonian candidates will upset the balance. How do you decide which of the 10 is allowed on the list?
- Put him on the candidate list but swap him out with another Elbonian non-religious candidate? That is an incredibly dangerous precedent. The religious Elbonian candidates could therefore improve their chances of being elected, if they can find a few Elbonians who are willing to run as non-religious on a particular platform that's so extreme and far removed from your country's political compass, that these new candidates pretty much guaranteed to not get any votes.
If you only manage your candidate list, you're effectively going to be striking a few candidates from the list. That is effectively a first election, one which happens behind closed doors. This again violates the core principle of democracy.
In a pure democracy, every candidate should be put on the ballot, every eligible voter has an equal voice, and the election result should be final.
Changing any of these parameters always infringes on the democratic principles.