Lineage has been the go-to ROM that beat every other ROM in active users by one or more orders of magnitude since the beginning of Android. Everyone that cares a little bit about custom ROMs knows what Lineage is, and if they don't they will recognize its previous name CyanogenMod.
Average users are never going to care about custom ROMs because it's not worth the effort for them, so why would you care about making LOS more mainstream?
More people using a thing (in this case I mean FOSS more than LOS specifically) influences the market, companies start to notice and may even be forced to adapt. Large private monopolistic ecosystem are really bad for everything.
It would make things easier for smaller brands to enter market as well - like what Raspberry/Banana Pi but for phones.
Also the official list of supported devices (wiki.lineageos.org/devices) has a super easy install process. Mainstream would mean more of these (eg the latest flagship Samsung supported is S10/Note10). And expanding this list would make a huge difference in addition - not everyone that knows about custom ROMs is tech savvy (or just wants a stable OS with OTA).
because adopting open source software is a societal good? the idea that it won't grow is kind of bleak. the industry standard for android device OS is dogshit for user privacy and a private monopoly and that's something we should want fixed. unless you like living under constant corporate surveillance.
I never said I didn't want it to get popular, I just think there's no point in trying to spread the word about its existence... because the problem is not in people knowing about its existence. The problem is that most average phone users will not find it worth the effort to switch to LOS, with the risks that come with it (banking apps not working half the time, upgrades sometimes needing a data wipe).