It's really hard to predict what may end up popular but you sure as hell can help guide the process.
Let's take a look at an all time favorite: Lucario.
Had appearances before Gen 4 started. Was the super duper ultra mega hyper legendary explorer™ in mystery dungeon. Was a main character in a very good movie where the Lucario itself had a very good character arc and tragic backstory and heroic death.
And then the Gen 4 games happened, and what do you know? You are given a Riolu egg mid-game yes or yes. Lots of trainers also have a Lucario, too.
That is a lot of exposure for a Pokemon, everyone will come to know it because of that. That alone is a massive boon.
And then you take into account other things like how it has a special snowflake™ ability in Aura.
Another thing worth mentioning is that Lucario is also a very good Pokemon in terms of stats, movepool, ability, etc. It's very usable, and during its debut generation it was a very good Pokemon in competitive. People really underestimate how much usability influences in a Pokemon possible popularity.
It would have been a miracle for it to not become popular.
Now let's look at Zoroark.
Has a cool unique ability... which was immediately made useless thanks to team preview and has next to none real... what is the word... well, it's pretty much useless during a normal playthrough.
How do you get a Zorua or a Zoroark? (Forget which one is that you're given) through an event. Hard to obtain. Barely has any appearances through the game and pretty much all the ones it has are with N. Became highly associated with just one trainer rather than something anyone could have.
And last but not least there's the movie, where it had an appearance before Gen 5 started... As bratty, snarky, bullheaded, annoying Zorua and a mom trying to find her child. It may be unfair, but let's be honest, not many people think of "cool" when told the word "mom".
And if there's one thing everyone agrees they hate about Pokemon movies is childmons like that.
You can see why one succeeded and one failed.
So basically...
>Lots of appearances through the various media, specially the anime, appearances where the Pokemon is on a role of importance and the portrayal is favorable
>Be good competitive-wise and in-game
>Be easily accessible and easy to obtain so that everyone can shove it on their party
That's the trifecta, I think.
Of course there's a lot of other factors to take into account. You can do all those things done with Lucario and do them with, I don't know, Rellor, and Rellor wouldn't be popular at all. Looks matter. And Rellor is ugly as sin.
At the end of the day not all Pokemon are made equal. There will always be surprise hitters, like, say, Dracovish, but at the end of the day when you go and look at which Pokemon are the popular ones and which ones are the unpopular ones the reasons as to why their levels of popularity are as they are very obvious at first glance.