The short answer is that we're rarely in a position to deem fixtures to be low priority, which is often where youngsters get regular runouts for other clubs since the domestic cups are usually treated as secondary until they reach the quarter finals (or their season's been a write-off otherwise)
That's the thing with Harry Redknapp sacking off the Europa League in 2011-12, it meant that Andros Townsend, Jake Livermore and Tom Carroll got some regular minutes to build off of, and we haven't really been in that position since as we're either in the CL and/or make such a pig's ear of it the youngsters get the same 90 minute window as everyone else as happened against Colchester a few years back
Realistically fans need to accept that if we're going to blood youngsters that means the Season Killer Cup needs to be low priority in the early rounds as giving minutes to Donley, Dorrington, Lankshear et al is most important at that stage of the season...so, of course, we inevitably get drawn away to Chelsea for the first game