For the same reason that ICF coaches are mostly restricted to 24 coaches: PF length in most cases means that beyond 24 coaches train will go out of PF. Since LHB coaches are longer, the number of coaches are lesser.
POH=Periodic Over haul. Just like evry house hold will carry out major repairs every decade or so to maintain its longevity, same way coaches and locos have to go major repair and renovation process periodically to keep them fit for mainline service. this is the reason why you'll see more than 25 year old WAM 4 blaze past at 110 Kmph :P