For us, there are so many different factors starting with what are we intending to get out of, or do on our vacation.
Our friends talked us into a 4-night Wish cruise in January at the last minute because they were going and we wanted to see them and they, and we, wanted to check the new ship out. They chose that particular date because it was the least expensive time, plus some scheduling issues.
I'm planning a short 4-day trip this September because I just want to get away and my schedule is limited. We'll probably do the Wish again, just DH and I, as we missed a few things and though we love sailing with our friends, when we do cruises on our own, we do them differently, and we'd like to try that on the Wish and see how we feel about it. We didn't get off the ship on the January cruise, and we don't intend to get off this time. So ports are irrelevant.
Sometimes we want to cruise new ports, so we look into what's available (we are considering a couple of other cruise lines for this but haven't made a final decision on that yet - still pondering it) and when. So that would be bounded by what's on offer (port-wise), then schedule. Then we'll decide if we like the ports enough that we'd jump to another cruise line if necessary. We like to see the world and had been routinely doing that every 18 months or so with a longer trip - cruise or land vacation. Covid has obviously interrupted that, but we're trying to resurrect that pattern. It's also possible that, if DCL doesn't go somewhere that we're interested in that we might default to a land vacation in Europe instead rather than changing cruise lines. No decision as yet, just collecting options.
Sometimes we just want warmth. I'm gonna go DCL on this for sure because I know what I'm getting, I know what I like and the ports are nice but not necessary and I don't really care which ones they are. It's a much less stressful vacation for me because if I miss something in the port, can't get an excursion I like or don't get an experience on the ship, no biggie. That will usually be selected for timing/schedule/need to get away from the cold. Sometimes it might be the schedule of our friends, so when can they get away. With that, the particular ports are even less relevant for us.
Price is usually the last thing to consider and the question is: do we think the experience is worth the price? So sometimes, if we have some flexibility in scheduling, we may shift the dates for what we think is a better-priced cruise if it still works with other factors.