It is true, Gladiators rarely fought to the death. You got to remember they where very valuable. Getting a guy to be good at fighting, good at showman ship takes a lot of work, and finanical effort. Cost of housing, food, supplies, etc. To spend all those resources only to have him die in the first fight?

Also many gladiators became celebrities in their own right. A director is more likely to kill Tom Cruise in a movie then the owner of a Gladiator, they are worth more alive then dead.

Also not all Gladiators where slaves, or owned by someone else. Some people choose this as a career path. Those people aren't interested in dying either.

If a gladiator dead in combat there was an extra cost added to the show to cover the cost of replacing said gladiator. Killing a gladiator wasn't banned, just expensive.

http://legvi.tripod.com/gladiators/id2.html

TL:DR they where expensive.