When the iPhone first launched, it wasn't adopted by everyone. I remember when, in ~2009, I and a few friends were the only ones I knew who had an iPhone. It wasn't until around 2011 that pretty much everyone I knew had a smartphone.
The basic business lesson is that you have to "cross the chasm" from early adopters to mainstream users - you can't just start selling a new product to everyone and expect to succeed without massive resources, and even then your product can still struggle (see: Apple Watch). Really, the only way to pull it off is to establish a niche market before targeting the general market. This book  does a great job of explaining the process - Mozilla and Canonical execs would do well to read it!
Sadly, many startups don't seem to have very good business sense and long-term thinking. "If we're not the iPhone yesterday, then we can't compete and we might as well 'pivot' and 'focus' on our 'core offerings'" - not true! There is a market for a user-respecting, fully (or even mostly!) FLOSS smartphone, but my parents certainly aren't going to buy one - they just got their first iPhone last year after all - so sell to me and people like me, not them. Then, maybe in 6-8 years, once the platform has matured and the FLOSS benefits become obvious, you'll start to break into the general market and start to challenge iOS and Android. But I don't think most companies are down for that; sadly, most are just focused on next quarter's profits instead of building something lasting.