Bitcoin simply isn't convenient enough for every day use. Some people (including myself) could say that Bitcoin wasn't designed for every day usage(despite what Satoshi was claiming in the whitepaper). The expectation of 1 billion daily Bitcoin users will never happen in reality and there's nothing wrong with that.
Nobody is saying that Bitcoin is supposed to be the number 1 global currency. I would be perfectly fine, even if Bitcoin never becomes truly mainstream.
I don't think that anyone could verify the accuracy of this statistical data, but 30 million people seems like a pretty good number for a 12 year old technology.
BTC reaching 1 billion active users seems more like a fantasy, rather than an achievable goal.