I think put Defold on Steam need some money, but that should be a good way to increase the popularity among gamers and hobbists.
Some game engine are on Steam already. If no plan on it yet, itch.io is also a good place to gather potential users (although itch.io is not so well organized like Steam).
It would for sure get more popular if stable versions of the editor were released on Steam as a free app. Easy win to boost userbase.
Would that really have an impact on the number of users? Would a game developer chose not use Defold because it is not on Steam? I find that hard to believe, but who knows, maybe that’s the way to download and use tools nowadays?
Steam has a viral effect with Steam presence and “like this” on the Store page. It would be nearly free marketing. Game devs are probably friends with other game devs (I know I am). Most developers I’ve told about Defold, even ones who work for other parts of your parent company like Blizzard, have told me they didn’t know Defold existed before.
Every streamer that notices the “Made with Defold” text on any of my games has remarked that they know little or nothing about the engine. Given that Google returns Defold’s website right near the top of the results when “2D game engine” is searched for, the lack of awarness puzzles me.
The Godot devs did a lot of stuff on Reddit and people love open source stuff so that may account for some of that engine’s popularity.
Some other tools on Steam, these all spread organically from use the more they are used the more they spread.
https://steamcharts.com/app/365670
https://steamcharts.com/app/404790
https://steamcharts.com/app/248170
Personally, I enjoy Defold’s lack of popularity. I think it’s part of the reason the community here is so good. Generally, only people who are serious about making games, appreciate solid software, have some experience with other game engines, and don’t care about popularity contests decide to use it. Defold isn’t hard to find. It shows up on most lists of game engines, and it seems like the user base is growing nicely (I could be wrong). If it suddenly becomes trendy, the main result will be a horde of idiots (pardon my French) causing trouble for everyone else. This is just my selfish and kind of misanthropic opinion of course. 
This reminds me of the Cogmind dev, who deliberately made it hard for people to buy his game in early access so that only committed fans would do it.
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Once Defold reaches Steam I’ll use the Steam version 100% of the time to help convince my gamedev friends to use it too.