RISC-V is an open standard under an open and free license. Which means that it doesn't require an expensive proprietary licensing fee. It is the necessary development bed upon which open source hardware can be created. Effectively it means that it has the potential of creating cheaper hardware that manufacturers can create with lower cost overhead and whatever improvements they make upon the designs can be used for free by other manufacturers.

The RISC-V ISA is free and open with a permissive license for use by anyone in all types of implementations. Designers are free to develop proprietary or open source implementations for commercial or other exploitations as they see fit. RISC-V International encourages all implementations that are compliant to the specifications. […] There is no fee to use the RISC-V ISA. FAQ

While all other ISAs are proprietary standards that charge chip designers up the nose to even look at the specifications. Hence why there's so few chip manufacturers in the world.