Whats the reasoning behind lux's E costing 30% more mana than most mage ults?

I'm poking around lol wiki and seeing everyones ult costs 100. Most regular skills cap out at 60-70 mana but Lux's max E costs 130 mana. The only mages I found that even got close to that were brand pillar of flame (320 base dmg .75 ratio) at 100 mana and viktor death ray (410 base dmg 1.1 ratio) at 110. Both these skills have high cost because their potential damage is large. Xerath also has higher type ratio spells and high mana costs but his passive is mana return.

Lux's E is just a standard ol .6 ratio AOE nuke with a slow. It makes no sense to me. Maxing at something like 70-80 is probably most in line with other champs. Even 100 mana cost would be a stretch. But 130? Why?

Edit: Lots of people telling me now good E is but I feel like a lot of what people saying isnt in proper perspective. It costs DOUBLE a regular ability and even 30% more than ultimates cost. Maybe it needs a longer CD if it needs to be gated but its hard to justify it being basically the highest mana cost ability in the game by a large margin.