Why is the standard of evidence required for a civil suit set lower than the standard for a criminal suit?
Look at it the other way around and it will make a bit more sense.
The standard of proof is “beyond a reasonable doubt” because the founding principle of our nation is liberty, and we do not feel that we ought deprive a man of that liberty absent the most compelling circumstances.
Think of the “baseline” as preponderance of the evidence. That’s good enough for property and other such disputes, for reasons of efficiency.
But we care more about liberty than we do about wasting the court’s time. We care more about getting the perfect result than we do about any other considerations.
Yes, but why should I be deprived of millions of pounds without the most compelling of arguments?
EDIT: I see you edited before my post.
You can earn more money. If life or liberty is taken it can’t be returned. You can be freed after serving a sentence, but that doesn’t give back the time you spent deprived of that liberty.
And since you are not in the US, I don’t know what evidentiary standards apply there or the reasoning behind them.
You could look at this way. Some civil suits start out as “tied” from the beginning. A divorced couple is arguing about assets and maybe children. Two people claim the same lost money. All you can do is have a preponderance of evident to declare who the winner is. There is no philosophical standard to go up against. This doesn’t apply to all of them of course but it is a start.
Very well put. If you must think of it as being “lower,” it is because the stakes are lower.
A criminal suit is Gov’t vs. people.
A civil suit is people vs. people.
The government is more greatly restrained from acting against the people than the people are from acting against each other.
That, actually, is closest to the truth of anything that’s been said. 
Actually, while this is relatively accurate in describing the legal logic behind the “preponderance of evidence” standard (IMO, at least), it’s slightly incorrect in one small but significant way. There’s always an “onus” in any court case – an initial presumption that must be overcome. In criminal cases, it’s that the defendant is presumed innocent. The prosecution has to prove him guilty beyond a reasonable doubt – if they completely fail to do so, the defense doesn’t have to do a thing, beyond move for a directed verdict of not guilty.
In a civil case, the onus of proof is on the plaintiff, the person bringing suit. Presumably the respondent, the person sued, believes himself to be in the right, or it wouldn’t have come to trial. Say he’s possessed of $500 dollars that plaintiff alleges is owed him. Plaintiff must first make his case that he’s entitled to that $500 – which respondent then gets to rebut in his own case. But, as noted, the standard of proof is merely the preponderance of the evidence – did plaintiff or respondent make a better case that he should have the $500?
The point, though, is that in each case you start out with a legal presumption, which must be overcome by evidence and argument – which itself can be overcome by other evidence and argument. In the civil case, it’s that respondent is entitled to what plaintiff is suing him for – pending plaintiff proving that he is in fact entitled to what he’s bringing suit for.
In an idealised sense, it is also merely a question of adjusting rights between equals, so each side gets the same proof requirement. The question of why someone should pay over a million bucks they say they don’t owe is balanced off against the question of why someone else should be deprived of a million bucks dollars they say they are owed. The two sides of the same coin involved match the even handed standard of proof.
Its good to get something right during finals… 
Consider that in a civil suit, one or the other party must be the “loser” regardless of the outcome. If you insist on higher “beyond a reasonable doubt” for one verdict, then you are of necessity setting a much LOWER standard (reasonable doubt) for the converse verdict.
If Joe has to prove beyond a reasonable doubt that he is the owner of the disputed chicken, then I have only to provide a reasonable doubt of Joe’s claims for the judge to award me the chicken.
on preview, pretty much what Princhester said.
[Tiny, editing nitpick]I assume you meant “-- if they fail to completely do so,” or “-- if they even partially fail to do so,” That’s what you’re saying in the rest of the post, and it does affect the meaning significantly.[/nitpick]
That and the influence of the English legal system which gave rise, for example, to Magna Carta.