So, let's begin with some overview of OpenRC.
OpenRC is not a replacement for the init binary, which is provided by sysvinit. Instead, it works in conjunction with it.
initscripts, systemd and OpenRC
Before 2012, Arch Linux used a custom BSD-Style init system called initscripts. But, according to the current set of Arch developers, initscripts weren't modern enough and were a chore to maintain.
There was a new kid in town called systemd, it was modern and provided a lot of features (all?) that made the developers drool.
The main reason for adopting systemd was — less effort on part of the developers as they no longer had to develop and maintain initscripts.
Another reason was that (theoretically) upstream developers would be providing systemd service files (required to run daemons and more) for their packages and hence distro packagers wouldn't have to write their custom service files. In reality, I don't think many software developers have started providing service files for their programs. But, this could change in the future.
Anyway, there was a proposal to incorporate OpenRC into Arch, but it was rejected.