Why don't people play Lissandra?

I just found that she's an absolute counter to one of the biggest and most played assassins: Zed. With basically a free Zhonya's at level 6 (plus a heal) and pretty bursty damage for a mage; I don't get why she isn't played more often. She can be quite tanky health-wise as well. So why doesn't anyone play ? (I haven't seen that many. Maybe like 3 or 4 in all)