The RISC approach to microprocessor design has been around since the 1980s, but the specific instruction set called "RISC-V" has only been around since 2010.
It takes years for tooling to improve and for industry designs to start making it out into the world. As more designs make it into end-users' hands, the tooling has even more motivation to improve.
I feel like that slow evolution can make it appear "suddenly popular" despite being around for a few years. =)
Design work was started on RISC-V in 2010 and an initial frozen spec was released to the public in 2015. The first proper 32 bit chip and board (HiFive1/FE310) shipped in December 2016, and the first Linux-capable 64 bit chip and board (HiFive Unleashed/FU540) in April 2018.