This question was answered a while back (stumbled on it by accident)
https://www.reddit.com/r/AskHistorians/comments/28gmvc/why_did_the_pilum_fall_out_of_favour_for_roman/
tl;dr: the plumbata has better range and is easier to use.
Edit: didn't know the username ping decorum, my apologies u/Ambarenya