Only applying some logic here, as well as opinion based on reading and experience--my guess is that terrestrial patterns took second chair to mayflies from the beginning because anglers were not at all opposed to using bait in the early days, and many terrestrial insects were both easier to capture and easier to attach to a hook than were mayflies, so they simply saw no need for imitations. Once fly fishing developed into a distinct sport, with "rules," terrestrial patterns may have taken a back seat because there were already mayfly imitations aplenty, and traditions tied to fishing various aquatic hatches.

I remember, growing up in the US in the 50's and 60's, that the fly rod was considered by many to be an excellent means of delivering lightweight, fragile baits, like worms, crickets, and hoppers. Bait fishing with a fly rod lost favor over the 70's and 80's, and by the 90's was forbidden on most trout waters, so interest in terrestrial patterns grew as anglers wanted more time on the water and aquatic insects didn't. The growing interest in midge patterns followed the same timeline, and likely for the same reason--they are more reliable hatchers, year-round (that, and the availability of thinner tippets and high-quality tiny hooks).
-CC