In general, a possible common mistake in reasoning on such topics is the correlation of the number of registrations with the growth of bitcoin. Perhaps these words will seem strange, but people come to this forum more often not because of the growth of bitcoin, but because of the alt season. It is the altcoin section that is the most popular and most densely populated among beginners.
Therefore, bitcoin has little to do with the growth in the number of users, both in 2018 and 2021. The catalysts for the growth of activity, posting and registration are bounties and altcoins. These sections are the largest and most active. And when another hype of altcoins occurs on the market, the activity on the forum increases.