[Milsurplus] ARC-5 Receiver and other tube questions

[Ray Fantini]

I always thought the metal case tubes were newer and better design then the glass tubes, the metal blocks inter electrode coupling, with a metal tube you don't need a separate tube shield. The old primitive BC-348 receivers used glass and shields where the newer series like the BC-348 Q used modern metal tubes not requiring exposed grid caps and tube shields. The ARC-5 stuff never evolved into anything requiring a redesign like the BC-348 did , the only thing I can think of is the late forties ARC radio equipment and think that all used metal tubes with the exception of the seven pin stuff.
Question, why did some tubes stay in glass envelopes like the 5Y3 and the 6V6 for so long? Was glass a better heat transferring medium for hotter tubes? Cooler tubes like 6SK7 are hard to find in glass after the early forties but 6V6 stayed in production in glass almost forever. I have never seen a metal 6V6, where 6L6 was first a big glass tube then a smaller metal one, are all 6V6 glass only? What about the 6K6?
Ray Fantini
KA3EKH