I'm not sure why that should be, Bub. In his British Patent GB 2 230 426 (now lapsed, if anyone wants to duplicate it), John Watson specifies that the frame be "devoid of tubular elements and closed cavities in its construction". He doesn't give a reason why, but then, in a patent, he doesn't have to - all he has to do is say essentially "do this and this and it works".

I presume that the avoidance of tubes and closed cavities must relate to the possibility of standing waves being excited in them, thereby causing resonance and unwanted vibrations. However, these occur only at certain frequencies, not right across the board, and sizing the tubes to avoid this possibility should be straightforward. (Edited to add: Missed something - In a quick look through the description of the drawings, he does use the word "resonant", so that's the explanation).

Tubes have the advantage of being inherently very rigid structures, much more so than the angle iron used by Mana, and therefore theoretically more suitable as construction materials for stands. A stand of similar rigidity as a Mana could be made much lighter. You will justifiably reply that the proof of the pudding is in the listening, but the Mana Effect may be due to other factors, such as the use of glass (specified in the patent) supported on upwardly-directed spikes. It would be interesting to hear whether a tubular stand built on Mana glass/spike principles would sound any different from your non-tubular version.

P.S. Glass features in the patent, but the black adhesive strips on the glass don't!