Lucian was overnerfed
In lucians old state, he was essentially just a better version of graves. He had more burst (This is debatable), more mobility, more range, less mana problems, and a better lategame. This was a problem.
But, after Lucians onslaught of nerfs, graves is essentially just a better Lucian. Graves now has more burst, less mana issues, a better lategame, Lucian still has more mobility and range, but not by much.
I do admit it is better now than it was before, as they do each have their own respective traits that they are good at now, but lucians numbers are just too low. After 4 patches of nerfs, its just too much.
Riot look at the statistics. After 4.21 literally everyone switched to graves, including myself. They are way to similar right now and as it is whoever has better numbers is picked almost all the time.