It's something to do with the agreements Google forces on the phone manufacturers. Android itself is free, but Google attaches various conditions to the licenses to sell phones with GMail, Google Maps, etc. pre-installed.
A pretty well-known condition is that Chrome must be pre-installed if any other Google apps are pre-installed. Previously they required that Google apps be the default handlers for everything if Google apps are pre-installed (Chrome must be default web browser, GMail must be default e-mail handler, Google Messages must be default SMS handler, etc.). The EU decided that the requirement that Chrome be the default browser is unreasonable years ago, so Android phones sold in the EU may have different default browsers.
(As an aside, although I don't use most of the Google apps, changing the defaults and uninstalling/disabling Google apps when possible, along with Facebook and the rest of that pre-installed crap, is a minor one-time annoyance for me when buying a new phone. You have to do the same thing after installing an OS on a PC after all. It's probably more of an issue for people who aren't tech-savvy.)
The majority of people are going to want to use at least one of the Google apps, and needing to install them after buying the phone is apparently too much of a burden for people who aren't techies. It's got to have that stuff out-of-the box. So the phone manufacturers will usually put up with whatever conditions Google demands. Samsung are big enough that they can tell Google to piss off if they believe the terms are unreasonable, but most of the other phone companies aren't in that position.
This brings us to the specific thing Epic is complaining about. Epic were negotiating with a few of the phone manufacturers (Oppo, OnePlus, etc.) to sell phones with the Epic store pre-installed. Google heard about this, and told the manufacturers that their licenses to sell phones with Google apps pre-installed would be terminated if they sold phones with the Epic store pre-installed. The phone manufacturers can't afford to lose the license to pre-install the Google apps, so they walked away from the deals with Epic, so there are still no phones with the Epic store pre-installed.
Epic's complaint is that attaching this condition to the licenses to pre-install Google apps unfairly stifles competition, as it prevents them from having their app store pre-installed on phones, raising the minimum effort a potential customer requires to discover/purchase something from them.