It seems the issue has more to do with birth and death rates, than any growth or decline due to the merits of the religion or practices:
It’s mainly down to births and deaths, rather than religious conversion. Muslim women have an average of 2.9 children, significantly above the average of all non-Muslims at 2.2. And while Christian women have an overall birth rate of 2.6, it’s lower in Europe where Christian deaths outnumbered births by nearly 6 million between 2010 and 2015. Religion: why faith is becoming more and more popular | News | The Guardian
I’d argue that with respect to Buddhism, there will be growth in parts of the world where science and reason are fundamental to the culture. For example, one article points to Buddhism being the fastest growing religion in Australia. Here in the US, Buddhism seems to be growing and inviting interest from individuals raised, for example, in Catholic or Jewish families. I’d also argue that it might be true that, while the sheer numbers of Buddhists may remain static, the quality of that Buddhism might improve, thanks to SC and other avenues where more people have access to Dhamma in more languages. This will clearly bring more inquiring people into Buddhist practice.
Some declines in Buddhism may stem from evangelization efforts in traditionally Buddhist regions. I have seen, for example, in northern Thailand, very active efforts by Christians with US based churches bringing education, healthcare, money and other incentives to Buddhist regions, and converting people there to Christianity. I find this to be tragic, insofar as many of these traditionally Buddhist families might be better served, no matter their economic status, living and raising their kids with the Dhamma vs. a “faith-based” spiritual crutch such as American evangelical Christianity, but this trend seems not to be reversible in some Asian communities.
So, if the quantity of Buddhists around the globe is not growing relative to other religions, due to birth and death rates, and coerced conversion issues, I expect that the quality will grow. I also see a real cross-pollination with other groups and factions in the west, such as the “mindfulness movement” and popular media figures (such as Sam Harris, Dan Harris and others) who all but speak of and teach Buddhist principles without using the word “Buddha.”
“Why Buddhism is not growing?”
I’d say Buddhism is growing, in ways that might not be as obvious or measurable as the Pew article suggests.