Percent damage reduction can easily get out of hand. In this game, the devs have traditionally kept it in check by linking DEF/MDEF to gear tiers, so that the amount of damage that tanks receive is fairly uniform.
You can have a lot of variation in secondary stats. You have to account for tanks in entry level gear with much lower Tenacity, and tanks in higher level gear with potentially much higher tenacity. If it scaled too briskly, you could have a fairly significant difference in the damage taken, which could make encounter design for different skill and gear levels much more challenging. Tenacity could be much more powerful if it was a primary stat, as primary stats are generally gained at fixed amounts with each tier of gear, and you won't have two tanks with different amounts.
Make no mistake, %DR stats in MMOs are very powerful, and devs are usually fairly cautious with them. For all of our interest in doing solid dps, if stacking %DR gave us big enough returns, we'd prioritise it in a heartbeat. This is the sort of thing that can trivialise tanking in encounters.