One of the reasons is because mullvad doesn't do aggressive marketing like nord and express VPNs. TBH, I've seen these two vpn providers everywhere lol. For instance, I would see them sponsoring the most random video on yt lol-- I do not mean buying ad space on yt.com but sponsoring a youtuber's video.

Mullvad doesn't do paid reviews, KOLs and affiliates. It's a part of their business ethics which I have big respect for. This may be anecdotal but, I wasn't aware mullvad existed until someone in the forum mentioned it.

I do think differences in features does play a part as well. For instance, mullvad doesn't offer dedicated IPs like nord.
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Marketing as much as possible is not a bad thing if I must be honest with you, I also have been seeing nord VPN advertisements on almost every where and corner of the internet, both on YouTube, X (formally known as Twitter), Facebook and so on, this only points out to one thing, and that is that they have a really high marketing budget available at their disposal, and also very much dedicated to their business.

I know quite a number of really good VPN companies that ran same level of advertising in the past, but currently have slowed down, in fact, I no longer see their ads anywhere, yet the company is still alive and perhaps, doing well.
One of such companies is Cyberghost, a VPN I've used in the past like 6 to 7 years ago and was really good, I used their free version back then, I recently downloaded their app and tried using the free version again but I learnt they no longer offer free VPN, you download the app, register and subscribe immediately, or just delete the app and look for another lolz.