Remember 155mm gets huge amounts of development funds world wide that near equivalent rocket and mortar shells just don't get (it was one of the arguments in favour of replacing 4.5" on RN ships, and not buying US 5", with TMF 155mm). So future natures, guidance developments, novel payloads are all available or far more readily available as a result. Think of submunitions, sensor deployment. leaflets or even long range deployment of UAV. They're all developed or getting developed for 155mm, not so for 122 rocket, 120mm mortar etc. Or any other calibre outside of larger MLRS rockets like GMLRS-ER.
155mm just opens up far more possibilities...and given production volumes is not going to be significantly more expensive than rocket warheads or mortar warheads. Also gives you a far better hard target capability as well...no need to develop and add a penetration shroud for a 120mm mortar warhead etc.
As for fuzing? We're moving over to electronic fuzing, so utilisation of traditional fuzes could be easily circumvented.
We're rapidly moving to a position where major nations will have the capability of delivering 1,000 155mm shells per day...so in the grand scheme of things its a rounding error...