The phrase "to reach the mainstream" means becoming popular and widely accepted among the general public. It refers to something or someone that was previously niche or specific and has now gained broad appeal and recognition. Full definition
There are also other reasons why it has not reached mainstream adoption just yet, some of which I mentioned in the above paragraph. (zhugeex.com)
This was also a decade where pants for women got more and more popular, even though it didn't reach the mainstream. (thefashionfolks.com)
But it also suffered from poor battery life, a confusing interface and a seriously bad reputation for violating privacy before it ever reached mainstream consumers. (tomsguide.com)