Executing Operation CHASITY on schedule requires the Allied armies advance out of Normandy on schedule. Note that when the expected date of the start on installation in Quiberon Bay was to begin about D+40 the Allied armies were still stuck in Normandy, with two full weeks to go until the breakout, and another two weeks until Quiberon Bay and the adjacent port of L Orient were secure. So, its around D+ 60 when the execution of Op CHASITY could occur OTL.
Ruppenthal 'Logistics in Overlord' describes how components intended for the Quiberon Bay port, had been diverted to enhance the already operating port of Cherbourg. This started in July, & as the battle in Normandy dragged out a increasing number of components were diverted to further boost Cherbourgs capacity. OTL Cherbourgs nominal peacetime capacity of approx 10,000 tons daily was boosted to near 20,000 tons daily, and in September briefly surged to over 25,000 tons daily. The on schedule execution of Op CHASITY implies the material used to enhance Cherbourgs port is not available & the discharge there is significantly less for August and September. Alternately there is not the full kit for the Quiberon Bay site available when its set up and its intake is curtailed until replacement items are provided.
The full scale operation of the Quiberon Bay site depended on a secure Brittany coast. However L Orient & Brest remained in German hands into September. The USN Port operations group were, in August, concerned about routing Allied cargo ships past these German enclaves. Both locations had long range coastal artillery capable of interdicting cargo ships not approaching well out to sea. The US Army was optimistic in August about eliminating this threat, but the port operations group thought otherwise. There was a related concern about the residual submarines and torpedo boats ported in Brest, L Orient, LaRochelle, & Bordeaux in August it was difficult to correctly asses the real threat from these German controlled ports. This contributed to the decision to cancel Op CHASITY. The US port operations leaders felt the near term intake August - October would be far below planned. It was judged better to use the material remaining of the Quiberon Bay facility to improve the operating ports.
The failure to secure Brest & L Orient on schedule had another effect. The utility in restoration of the Breton railways was based on the total intake from all the Bereton ports, including Quiberon Bay. With the others not operating the efficiency of early restoration of the Bereton railways was questionable.
Port capacity was not all the Allies desired in the summer & autumn of 1944. However flexibility, improvisation, and brute force in engineering and industrial capacity kept the port capacity close to caught up to events. The supply problems the Allies experienced in 1944 were largely one of transportation from the ports to the battle front. The collapse of the defense in August and the high speed advance to eastern France and the German frontier set the supply transport distance far ahead of that planned. The severe damage to the French railways west of Paris, the poor condition of the rest of the Franco/Belgian transportation system left the ports in use clogged with discharged material that could not be cleared fast enough to the forward depots in the battle zone. As late as December this port clearance problem was still existent. Mid December discharge onto the docks of Antwerp was halted as the docks and loading yards were clogged with material that lacked transport. The railways, canals, and automotive roads were still not operating at adequate capacity. It took over a week to clear Antwerp of the backlog.
The transportation problem alone waives away any difference a on schedule of later execution of Op CHASITY. Then theres the question of the Quiberon Bay facility achieving its planned intake with the other problems of diverted components, and German interference.
Hope that clarifies things. Its a simple outline of the problem & missing many details. I'd recommend reviewing Ruppenthals analysis 'Logistics in Overlord' for a broad overview of the US Army logistics in western Europe.