In order to enter the mainstream market and challenge the consumer OS duopolies, a new OS needs at least two things:

1. Retail presence

2. A large advertising budget

This is why it's so difficult to challenge the existing duopolies on desktop and mobile. If a consumer can't walk into a retail store, see a device on the showroom floor with the new OS installed by default, and buy a device with the new OS installed by default, then the new OS has zero chance of becoming mainstream.

Among other reasons, this is why Linux has failed to go mainstream. Linux has no retail presence, and it's not advertising to consumers.