That would make sense, being a logistical decision more than one rifle outclassing another, especially on a wartime footing.
When I look at the history of "Full bore" rifle clubs in Australia, the P14 has always been permissible in competition ( and in later years, the N0.4 rifle), but the "H" barrel SMLE continued as the mainstay for decades, so lithy's point is quite valid as well. I have owned a number of "H" barrelled rifles in different configurations, and all have shot very well. They do need varying levels of tinkering to get the best out of them, but that can be said for most competitive pieces of equipment. I do remember a number of comparisons made in the '60's between the 7.62 cartridge and the .303, but the results always appeared to be inconclusive; handloading wasn't allowed, and stock military ammunition will always be a variable.
There does appear to be a huge amount of misinformation out there regarding SMLE accuracy, some of it hearsay from younger shooters, and some from older shooters who didn't know how to set up the rifle correctly. Poorly sporterised No 1 MK 3's did a lot of damage to their reputation.