Reading this thread was enlightning. Wondered if the chain was desired to overcome manufacturing tollerance issues to some extent perhaps as well since the head gasket will compress to different heights depending on several factors. Centerline of camshaft distance to head face may vary a little as well in production. A gear drive train I suspect would have to incorporate some method of adjustment to get all gear lashes accurate possibly because of production variables. Just maybe hard and difficult to engineer properly. Perhaps the chain also cushions shock loads a little better than a multiple gear train would.

dont know
 

about all that re clearances etc. but i believe that many race engines have all gear drive for overhead cams so i think that a chain is probably mainly a cheaper alternative and less noisy.