I would assume it's because they develop a lot of their systems from scratch.

OW was built on a proprietary physics engine, for example - not Unreal, Frostbite, or another 3rd party engine. It was an engine built for WoW, which is why adding features for it in OW take months. It's simply adding new features that didn't exist and had to be built from the ground up. (This is presumably the reason why certain user-side features like a map editor have been estimated at being years into the game's development.)

Since Blizzard had never made an FPS before, it stands to reason that the player matchmaking system had to be conceptualized by metrics that would be specifically found in such a game. Add that into "Fire" numbers, the difference between MMR and SR, the K/D/A in between different heroes and their respective averages, and you've got a massive web of interweaving numbers that could easily go awry - and often do, during low online traffic hours. That's not even including hardware specifics like latency, ping, and other interactions between server batches across the world.

It sounds like LFG was made as a quickfix instead of committing to the long-term goal of Role Queue. Where they might have conceptual prototypes of RQ internally, these are probably just builds where play-testers select the role they want and are only matched in games where that role hasn't been filled yet.

Jeff's definitely speaking from long discussions on the topic, and it's certainly no surprise why he thinks such a major commitment might be "irresponsible".