Well the short answer is politics.

The long answer is that when we had the opportunity to make a change, during the ERRE committee time period Trudeau was extremely concerned with presenting himself not just as a Harper alternative (IE a "Not Harper PM") but as fundamentally different from him. Harper had tight, party whipped votes and was known for being very singular in purpose, he got done what he set out to do like it or not (And I did not). Harper did not collaborate he pushed everyone around. On the other hand, despite having a majority government Trudeau set out to govern more with consensus and collaboration, even going so far as to, under some pressure, give up majority control on the ERRE committee at the request of the NDP, making the committee proportional (this decision proved to be fatal to ER).

It was in that committee where all parties essentially refused to budge on their positions and would not negotiate with the LPC to pass recommendations that the LPC could get past both the house (in a free vote) and the senate (which was much more conservative in 2016). The NDP wanted PR so bad, and STV/Ranked Ballots so little, that they sided with the CPC who wanted to kill the entire thing with referendums instead of working with the LPC to get some change through. The result was recommendation that absolutely had no chance of passing the house, and if implemented no chance of passing the senate, and even if passed through both would not have resulted in reform before the next election as a referendum killed that idea entirely.

So rather than act like Harper would have, and use his majority to push through STV over the objections of the other parties, Trudeau chose to drop it. Instead focusing efforts on things he could pass, like the Elections Modernization Act of 2018 and the removal of senators from caucus.