Source Memes

Mostly because L4D was mainly Valve's work, showing off the tech they are capable of. Turtle Rock Studios might be a good studio, but what makes them different to Valve is that Valve is also sort of a R&D company as well.

If they had the same base as Valve, they might have created a great spiritual successor to L4D. Yet in the end they didn't focus on the technical side, most likely due to publicly traded companies often trying to cut corners on anything that isn't directly marketable, a downside that Valve does not have.