Literally the entire reason why she's such a faceroll elo mage right now
I really don't think it's 1 thing making her an amazing pick.
What's making Lissandra amazing is more her entire kit does exactly what you'd want from a season 2 mage, offering a really solid mix of damage and utility without going to far in either direction (very much like
, who was a dominant pick in season 2 and beyond).
The main reason Liss hasn't been amazing until now is assassins coming into mid-lane during season 3 really screwed up the priorities of what makes a good mage, mostly pushing it to a Zigg's style of "can lane against an assassin" over having a really impressive team-fight presence.
Now that pretty much every assassin has been nerffed back to where they were in season 2 (not great), you're seeing what's made a solid mage in season 2 coming back in full force.
Add to that traditional supports and fighter items have been nerffed enough to allow mages a solid chance in top lane or as a support, along with the MP/5 nerfs to everyone that make Lissandra's passive not completely useless, and Riot's basically geared the entire meta to make Lissandra a good champion.