The polling questions aren't very neutral :p . Political science and psychological research has shown the way you word a poll can play very large factor in how people vote.
I'm old enough (very just barely) to remember when we had this debate before. The republican movement fell apart because it was a two stage vote. First, we had to decide whether we wanted a republic. Second, we had to choose which particular republican model we wanted, this is where it splintered and fucked up. We couldn't all agree on one we liked well enough.
I think a lot of the pro-republican people, including some of the people who voted i this poll. aren't aware we'd also need to choose a model and it might be a model with repercussions they dont like or understand.
In almost all cases, it will be a "feel-good" change. It will produce very little measurable, beneficial increase in our lives. Everything will continue to work the same as before, the streets won't be paved with gold if we get rid of the Queen. Your life will be exactly the same. Thus, we have established, that beyond emotion, there are very few tangible positives.
Let us consider a few (of the many) negatives.
(1) It is extremely costly to have a single referendum, let alone multiple ones. Im talking hundreds of millions when every single cost and dollar spent in the process is considered. And for what? Very little benefit?
(2) It steals political oxygen away from the important issues. Again, i remember the last time this popped up. There were TV ads, debates, everything. Our society and the world as a whole is on the precipice, we are staring down into the abyss and its a long way down. We're teetering on the edge of another financial crisis, we're struggling with a generation that's going backwards compared to their parents in living standards, attacks on most public institutions like medicare and the social safety net and so on. Is now really the time to have an issue of little practical consequence steal so much political oxygen and public will away from politics when we need it most? I say no.
(3) The cost of a republic itself and the model. Coins, name plates, stationary, it'll all need to be changed, at a massive (albeit, one off) cost, again for very little benefit. Secondly, we very much struggle to pick an appropriate model. Some people want a total shake up, others just want a very minimal change. I think those who want large changes are playing with fire, our political structure works very well. Minimal changes like swapping the name of "governor general" to "president" wont cause any harm but points (1) and (2) still apply, especially considered this is a tiny change for all that effort.