[kjaer]

People are more likely to already have npm installed and to be familiar with it. So there's an argument to be made that all else being equal, picking npm lowers the barrier to entry for new contributors. This consideration could be especially important for open source projects.

[moogly]

That's a valid point, but I don't think the barrier is particularly high. I've done the switch from npm to yarn once. It was a process measured in hours to understand the differences. It's not like Git vs Subversion or something like that.