Cosmo D: There have been suspended subchassis direct drives - Technics made some of them, the SL 1600 and, I think, the 1300, maybe more. Very good sounding tables.
Platter vibration is a matter of the design of the platter, not the drive system used, or whether or not there is a suspended subchasis. All platters have some resonant signature - it's the nature of physical objects -- but those that have a thinner inner section and a thick periphery are functionally bell shaped, and will ring unless damped by the mat or constrained layer or some other means, more or less successfully. Platters that are flat discs have less ringing, but they are an unattractive choice -- a lot of extra mass, which increases material costs and shipping costs, and if the motor has to be under the platter, the plinth will have to be taller, and the box bigger, again increasing shipping costs.
People will argue themselves blue in the face over whether compliant feet and/or mass loading are as effective as a suspended subchassis, but the reason for either is environmental - the sound field the turntable is in. Gross acoustic feedback is not the difficult issue, it's the subtler smearing muddying of the image and of frequencies. Suspended subchassis put the main resonances at higher frequencies and higher Qs than compliant feet and/or mass, and it may be that different ears hear the effects differently, and clearly, some don't hear the issue at all. Mass loading certainly is a materials and shipping cost factor, and different designers may choose to avoid that problem by using a suspended subchassis, which, of course brings it's own set of costs - parts, tuning, isolation from footfalls, and so on.
My point is that anyone making a turntable for sale isn't just trying to make a good sounding table, but is also trying to make the best sounding table for the price. Different designers will choose different paths, but none is inherently perfect, and none inherently perfidious. It was a mistake for some in the audio world to condemn all direct drives (and, while we are at it, idler drives) but it is also a mistake to anoint any system, but especially one example of that system, as the best for all situations.