Flat rides come and go in trends just the same way certain models of coasters do. In recent years we've seen the boom in popularity of Star Flyers, Screamin' Swings and more recently the Air Race... Before then we had the years when HUSS were king, not so much now!
Also with the rise of the 4D coaster, and more coasters which aim to combine flat ride movements into a coaster design, some parks must see this as a more marketable and attractive option instead of just sticking in a new flat ride. When parks are adding flat rides though, they do seem to be acting more 'safe' these days, with a lot of duplicates, but this happens worldwide when everyone knows that a ride is reliable, can draw a crowd, doesn't break the bank to purchase... etc etc. It's a similar story with the US carnivals & fairs, if you look at KMG's Facebook page they post updates about what rides they are constructing regularly. It's interesting to notice how they are selling more single-trailer rides such as the Speed (Booster) to showmen instead of multi-trailer rides.
The flat ride market is full of choice right now... I think it's just the case of building up trust with manufacturers and parks becoming more able to market them to appeal towards the guests. Naturally though it tends to be taller thrill rides such as drop towers and swing towers which parks are choosing because they can market them with raw facts, 'tallest in...' etc!