Basically vet dungeons, outside perhaps of a few DLC hard modes, aren't that diffcult. The pressure faced by a PvE tank is also simply different from PvP. As a rule of thumb, PvE bosses can hit really hard, much harder than a PvPer, but they do so infrequently. A PvPer or a zerg, by contrast, will pressure you with lots of small damage all the time, or at least as long as you're being focused, but their eventual burst isn't as a high as that from a PvE boss. A PvE boss' unmitigated burst, e.g. a single hit against a naked player, can be in the 100K to 200K range. This never happens in PvP.

The result is that PvP tanks and PvE tanks are different. A PvE tank needs block mitigation and some magicka sustain. Block cost isn't much of a factor, as long as it's below 1K. In PvP, on the other hand, block cost is a huge factor, at least for a blocking troll tank. An average PvE tank can perma block quite a lot, even when not specifically building for it. In PvP, if you want to be unkillable via blocking, you have to invest more heavily into block cost reduction. PvPers may charge you the block cost on cooldown. That cooldown is 1/4 second. In other words, you may incur the block cost 4 times per second. This almost never happens in PvE, but quite easily in PvP. Aside from that, PvPers have armor penetration against you, whereas PvE bosses rarely do. PvE bosses / mobs also can't crit and they don't CC you as often or as strategically as (good) PvPers. In PvP you're looking for crit resistance, line-of-sighting is important, breaking free, and so on. Tanking in PvP and PvE really isn't the same.