The most popular stable coin of Tether USDT became very popular in 2018 among the traders when cryptocurrency falled.
When cryptocurrency market falled traiders invested money in the most stable cryptocurrensies such as Tether. That is why stable coins became very popular.
I think that stable coins are very popular in any market not only falling. Stable coins are backed by real assets. That is why they are stable and the price of the coin depends from the price of assets.