New to guardian
Mostly, it’s because of the proliferation of condition removal skills, and the way burning stacks (duration rather than intensity). Additionally, we don’t really have “covering conditions”—that is, conditions we can apply to “mask” burning so that it won’t get removed by the first condition remover anyone in the area pops. The only conditions Guardians really have access to are blind (which is nearly always removed immediately, since the next attack the target makes removes it), vulnerability (applied on blind, or if an enemy decides to stand in a symbol for whatever silly reason), or immobilize (which has very short duration, and is only available on the scepter and hammer—also note that the scepter is looked down on in many PvE scenarios due to its single-target nature, and in any form of PvP due to its effective range being limited to around 5-600; any further away than that and people can literally walk out of its range before being hit, or sidestrafe a tiny bit to watch the little blue tennis balls go whistling off into the distance somewhere).
Anyway, with burn being our only damage condition, we can’t output very much damage with it alone; certainly nothing like what you can do with an equally-durable power-based build. A bleed-on-crit sigil would help, or perhaps a poison-on-next-attack-after-swap sigil, but we’re still not going to get anything like the coverage of conditions that more condition-based classes can get, and all of our effort that we put into maintaining burn uptime will almost certainly go to waste the moment someone pops a condition remover.
Another main facet of condition-based builds is their ability to lock down targets via debilitating conditions—poison to prevent healing, chill/cripple to lock down movement, fear to control movement and/or interrupt key skills, that sort of thing. We can give someone vulnerability… if they sit in a symbol, or we use a blind skill on them.