Because in ruins, you don't have to get into base to base to fight close combat to account for situations where you're assaulting a unit above you but there's not enough room to get even one of your models in base to base on the floor above.
Consider the following, lets say for example you have a mob of 30 ork boys encircled entirely around the edges of the pad, By the rules
GW gives you for the pad you basically can't assault them. The top counts as open and moving on to or off of counts as difficult terrain, however enemy units are impassable, you'd have to move through the orks to be on the pad, and thus in base, however you can't do that and as such you can't assault. Short of models levitating there's no way for it to work.
Note: I'm not advocating being a rules lawyer or that guy or just divisive but it's an issue.
GW has never been good with climbing mechanics or rules that fit well with tables with built in topography (think cliffs, steep surfaces). Hell, only recently (6th ed) did they fudge the base to base requirement when fighting close combat across a barricade. I'm sure plenty of people in 5th had no problem overlooking the base to base requirements when fighting across barricades (low cover) but it underlines how important it is to discuss beforehand rather than fall back on terrain rules that are at best a guide on how to define any number of terrain pieces,
gw or other.
I myself take issue with the skyshield because of all the potential issue that can come up, that and the idea that a tank can just drive up onto one or off of one seems silly. I've seen shots of like 3 manticores on one and it just left me shaking my head.
If
GW had gone further with the furled and unfurled stuff they could have fleshed it out more. Per haps furled (4+
inv) you can't move up onto ir or off of it unless you're a skimmer/jetbike/jump infantry and unfurled you can move up and down as it describes. And of course they could keep the reserve benefit for unfurled. The problem in my limited experience is that everytime I see one used in a batrep it's taken as a firebase and not as a landing pad.