I know that
Pretty sure they said it was because i130 is still a bit high for story content in Heavensward (at least til half way through it) which doesn't make much sense imo considering everyone playing on their mains will probably be at i120 - i130 anyway. Only people who will be weaker than that are people who were jumping at the new jobs straight away so they can go through Heavensward content on those new jobs.
Keep in mind that as of now the highest ilvl dungeon gear is ilvl 100. It'd be weird for them to skip to ilv 135 gear or whatnot during the first story dungeon; most players wanted to be able to use their gear for the first dungeon or two anyway.
So, first dungeon will probably drop ilvl110 gear. Second will drop 120, then 130. If so, we won't see armor that is a definitive upgrade until the 4th or 5th dungeon, (probably the 4th.)
Thing is, in those first few dungeons, ilvl probably won't be as helpful, since you're still going to be a few actual levels below the level cap for these dungeons. Between that, the dungeons being unfamiliar, and these being story dungeons, (More mobs for leveling purposes, no real way to vastly outgear content, etc,) these dungeons will probably be more challenging than the bulk of HM's we've faced.
So, there's some incentive to get the better ilvl gear for these dungeons, but it's not necessary. You'll be able to tackle them with ilvl110 gear, if albeit more slowly. Overall it's probably a fairly decent design decision.
The only real downside is that getting ilvl130 weapons requires a lot of Poetics, and I can see some people valuing those weapons over any armor piece. Since the next highest ilvl weapon outside Relics is only ilvl115, I can see why the Poetics cap would be annoying. However, a better solution would probably be to reduce the number of Tomes needed to buy these weapons.