What justification is there to keep facts from a jury?

Shows like Law and Order and The Practice are rife with cases where critical pieces of information is kept from a jury on the basis of a technality leading to either a false conviction, or the release of a guilty man.

One disturbing case involved a man who was convicted of choking his girlfriend to death during sex. He has been charged with this three times before with different girls. But in the previous incidents, the conviction was either sealed information and thus not admissable in court, occured during his juvinile years and was thus not admissable in court, or he there wasn't enough evidence to start a trail. Thus the jury being ignorant of all these previous occurances felt that it seemed unlikely and let him go to do it again.

I used to hope this was a limited occurance or one limited to tv shows, but facts tell me otherwise.

Martha Stewart's recent conviction for lying about something that wasn't illegal is just one example.

A recent article I read entitled Kobe's Beef oddly enough from Maxim presented a mountain of factual information that is critical to either the prosecution or the defence that the jury will not be told because of one techincality or another. The evidence from both sides is very significant but none of it will ever reach the jury.

In addition, the jury can't even be told that since Colorado makes no distinction between serial rapists, child molestation, and incidents of date rape, a conviction of a class 3 violation will land Kobe in jail for atleast 20 years without parole, and likely for much of the remainder of his life.

So I ask again, what possible justification is there for simply not presenting all the established facts to the jury, all the information regarding what these facts could mean, and letting them make their own judgment?