Excalibur reduces ammunition consumption by about an order of magnitude against a known location target, similar to Copperhead. Terminally guided weapons like Copperhead II would allow hitting moving targets with similar accuracy and lethality, but the only moving-target PGM is (was?) the SADARM. Maybe America bought BONUS. These may or may not be effective against roof ERA, but we know that EFPs can be stopped by armor protection (SPz Puma has EFP protection IIRC, as did XM2001) along with ICM bomblets of the 40-60mm class like M85 and M46.
The ammunition is heavy sure, but there isn't much a reason to fire 100 rounds a day, if 10 will do a similar amount of work, and 155mm rounds aren't 10x heavier than 105mm ones, rather it's much closer to 2-3x.
The newest 105mm is the M1130 base bleed HE, which lacks GPS guidance or terminal guidance. IIRC the PGK Increment that was supposed to be able to offer 50 meter/20 km accuracy with the 105mm was paused or something. The Army has only purchased 155mm M1156s, but they may be dual-caliber and just issued to 155mm gunners out of hand, as I don't know what the M1130's fuse well is like.
Other than that, there's no gliding-type munitions. 105mm is pretty weedy as it is and unlikely to support a large payload.
Since there's no Excalibur-type munition, let alone a SADARM or Copperhead, for the M119, the 105mm is consuming more rounds per target. Which means more barrels and general maintenance. Even if the individual ammo loads are smaller, the consumption of miscellaneous equipment probably balances out the payloads lifted.
On the other hand to new Zone 7 charges (for the M119) propellant grains look like jelly candy donuts, and gives the U.S. a supercharge-alike 18 km range with the 105mm like the L119s. Maybe barrel consumption isn't as messy with that as with the L119? I dunno.
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Forbidden apricot jellies.
Either way the main contributor to why M777 is better than M119/L119 is the use of precision munition natures of ammunition. If you had a stable of in-production precision guidance shells, like an Excalibur shell and a STRIX-type anti-moving target round, with a FASCM cargo shell or something, you'd be in a good deal of business of closing the gap and maybe 105mm could eek out the mid-range between 155mm and 120mm. But AFAIK no one has done this yet (yet).
There are certain PGK kits in catalogues that can be used in both 105mm and 155mm (this means the 105mm has a 155mm sized fuse well I think) and obviously people offer these things at trade shows, but AFAIK no one has taken up the deal yet. Most people seem to have either displaced 105mm or treat it less as a howitzer, and more as a mortar, due to its generally sub-20 km range.
PGK wouldn't solve the range issue of lacking a gliding/coasting munition like Excalibur, but it would make 105mm competitive per airlift with Excaliburs by weight at least. So they're still jockeying for position. IMO 105mm of typical variety (L119 or M119 or similar 15-20 km piece) makes sense for a battalion field gun because they're generally in close contact and don't demand massive gun ranges as a result. Stuff like G7 can close the gap in range, but now you have a 4-5 tonne gun that requires half a dozen Joes to man handle due to its limbers and barrel length, and you still don't get the good cargo rounds.
155mm remains caliber king of the division gun park for now.
