ORIGINAL: Kweasel
There must be a bunch of OS fourstrokes stashed away in closets. There's nothing wrong with Saitos so it must be the demographic of its owners. Saito has been upgrading their product line much more aggressively than OS, owners may want the newer engines. Or Saito is simply attracting more new flyers who are still crashing a lot or leaving the hobby.
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The younger crowd have responded to Saito's NITRO marketing ploys and they associate Saito with 3D performance. Of course, this is just my opinion.
I think you see so many for sale because the newbies trying to fly 3D do not understand the implications of producing good horsepower while burning lots of nitro with a very light engine (vibration and the attendant fuel foaming problems - especially with profile models).
I have many recently made Saito singles myself, so I'm not anti-Saito, nor am I bashing Horizon for employing such a successful marketing campaign.