Another factor is the length of the crankshaft. On an inline 6-cyl, the crankshaft tends to be longer, and therefore subject to more torsional stresses. This limits the max RPM's that an engine can be run at. This doesn't seem to be as big a factor on engines being built now though. They must have the metalurgy, as well as crankshaft balancing, developed well enough now. The old 6-es of the Fords, Chevies, ect, would tear themselves apart if you tried to run them above about 4000rpm (I know about this first hand
) I don't know this for sure, but I'm thinking that Datsun led the way of higher revving straight sixes with the 240Z's.