Why did 2 of my large lilacs die? And what do I do now?

I would advise to check for a few possible culprits:

check the bases of the stems for signs of chewing- could be a sign of lilac borers or scale insects, The problem might be because of poor drainage. In which case- replant with better soil, try amending the soil in a fairly wide area, rather than just the planting hole (if you confine amendments to the hole, it can act like a sink and draw water from the badly-drained surrounding area), and plant the root ball a couple of inches higher than the surrounding soil, creating a little mound.

Good luck!

I had 1 Lilac bush and it never made it either. I LOVE lilacs, they smell so beautiful!!! Maybe my junkie soil in my area. I know there are different lilacs for different zones. I should of asked for advise.