You just have to take a look at the global aviation market to find the most important answer. The passenger numbers in this decade that will end in just over a week just went through the roof.
As an a.nut site, we obviously focus on the airplanes. But I think what we are witnessing is more about the demand and ability of the airlines to employ A321 sized planes, than simply the improvements in the airframe.
We now have enough flying public that A321s are the right size for many routes. We have airlines that are either P2P LCCs or well-organized legacy airlines feeding their hub with A321-sized routes.
Airbus certainly have built aircraft that anticipated markets to some extent, with all the risks that can bring. The A380 was predicated on a long haul boom, partly to stimulate that boom and partly to wait there for the market to catch up - which it didn't.
The A321 has been there a long time waiting for the market to reach it. Carriers like Air France and Lufthansa could justify early fleets and rightsize the equipment throughout the day to satisfy demand (they were already running A300s on short-haul routes), but the A318 and A321 were the extremes, used in the rush hour or in slack times, as it were, or on heavy routes. Then along came the low cost carriers, boosting the demand for seats and creating an expectation that we could fly as a viable option, and the market went nuts. The market we have now is what the A321 was waiting for, and it's now beginning to look as though the A321 could usefully have a few more seats still.
It seems though that the low cost carriers are the ones taking most of these, Air Transat is using them across the pond into Gatwick and Paris, for example.
The natural competitor, the 737-900 was hobbled by its low ground posture making it a runway hog on departure, something the Max was going to resolve, and it might be factors like this that made the A321's day come a little sooner than it might have. Right now it's pretty much the only game in town and for some carriers, fleet decisions can't wait any longer than they already have.
The A321 also has the advantage of having been engineered from the outset to be as adaptable as possible, taking new engines with minimal design change and making the NEO option relatively inexpensive to engineer and quick to get to market. I get the definite impression that Boeing was caught somewhat flatfooted by the NEO and pressed by the need to create a more modern platform in almost no time at all and at minimal cost, we ended up with the MAX because a response was necessary, and soon. In the runup to the NEO Airbus talked a lot about a replacement (NSR) rather than an adaptation, and I think most of us were lulled into the sense that a new aircraft design was coming along, rather than a quick'n'dirty adaptation that turned out not so dirty after all. I certainly thought Airbus was looking at a clean-sheet design, from all that they were saying - not that my impression means anything.
I'm reasonably certain that had the NEO not come along, the MAX would never have seen the light of day and instead Boeing would have had their replacement in the air by now, a clean sheet design dispensing with all the legacy problems inherent in the old 737 design. As it was, the extensibility of the A32x platform inherent in the design gave Airbus the opportunity to take a lead at minimal cost, knowing that the competition would have more of a re-engineering job to do at a time when the 787 program was creating a lot of financial pressure. The MAX really was the only viable response given the conditions at the time but what it seems to have done in exacerbate the problem for Boeing - but then again they couldn't afford not to respond to the NEO either.
If the MAX had been free of its problems the A321NEO would have a stiffer fight on its hands, but I think for the moment the playing field is definitely slanted in its favour.
Short answer: It was always there waiting to benefit from an expansion in short haul travel, but the advent of the NEO and the frantic need to respond to that development created the conditions that have put it even further into the running.
These are just my impressions. They shouldn't be mistaken for firm opinions or facts!