If you cite Pokemon’s success or games like Hearthstone you need to consider the natural audience outreach based on the title history, or the company behind it. If you look at Battlegrounds, they made something with Unreal and basically did this all from scratch, they just delivered a very solid gameplay experience. And while many may have picked Unity in the past because of titles like HS for example, with new hyped titles developed with Unreal, there is likely more interest for development rising too.
And what does this mean, most popular, by audience reception, gross revenue, or number of developers, titles published, assets on the market, job postings is not an accurate measure.
In my opinion it really doesn’t matter, you can make great games with both engines. And then the fact you have to pay for the professional Unity version. I rather buy assets from that money. Today, I also consider Unreal Engine to be more capable when it comes to graphics, something which wasn’t initially a decision factor.