> If you get a ton of citations, who cares what journal it’s in?

Citations take a significant amount of time to accumulate (and not all citations are equal).

The way academic incentives work, people generally want to evaluate published work sooner than that, they don't want to wait for five or ten years to see how much the paper will get cited, and they do not want to judge the actual papers (since most papers in the world or even in computer science are outside of anyone's field of expertise), so the main proxy is the selectivity and impact factor of the publication venue, so people absolutely care about what journal it's in, because they will judge your publications primarily (or even solely) by where they're published, assuming that the journal's review and selectivity is more informative about the quality than whatever they can gleam from a quick skimming of your papers.

Generally there will be some sort of semi-objective criteria - e.g. they'll look at Scopus or Web of Science citation counts - ignoring any citations from anything else, and see whether that journal is in the top half of all the journals in that field (for example). And then some evaluation (e.g. for fulfilling research project goals, future grant proposals, or academic job evaluations - in essence whenever funding is involved, the funding officials defer to such criteria) the publications which fit those arbitrary criteria matter, and the rest are worthless.

The second half of the incentive issue (problem?) is that since you can't publish the same research twice (at least not in respectable venues), if you publish it someplace that "doesn't count" then that research work may be wasted for whatever metrics your institution or advisor is evaluated on, being worse than unpublished work that at least has the potential to become a "proper" publication. So my guess is that simply that "more scientific journal" fits those criteria better than the ones you mention, and your advisor is simply aware of the "metrics politics".