Riot excluded Lissandra from Mana Regen buffs

Exactly why did riot exclude Lissandra from receiving the mana regen buffs with the rest of the mages. I could only think it is because her passive gives her a free spell cast but when you look at Xerath and Swain and maybe even Veigar they all have arguably better mana regen passives than her. Meanwhile Liss is now stuck with a stifling 5 mana regen +0.4 per level. Why rito why?