Because of very long supply lines, and having no airbases in the region. The British forces had to try to project power sufficient to retake the Falklands over a distance of 12,000km, going up against an opponent operating only 700km from its mainland home. For any global military at the time with the exception of the USA and (possibly?) the USSR, that was a very tough ask. Some analysts thought it was an impossible task, and recommended that the British seek a negotiated political resolution to the conflict.
The other thing that made things difficult for the Royal Navy was the Argentines’ possession of numerous French-made Exocet missiles. These air-to-surface ship killers were very dangerous indeed, and the RN had no easy counter to them. Exocets were responsible for the sinking of HMS Sheffield and the supply ship SS Atlantic Conveyor, and heavy damage to HMS Glamorgan.
The destruction of Atlantic Conveyor made things particularly tough for the British forces. There were several helicopters aboard Atlantic Conveyor that had been earmarked for transporting men and equipment from their landing sites near San Carlos on the west side of East Falkland to tactical positions closer to Port Stanley. All but two of these helicopters were lost with Atlantic Conveyor, and instead the soldiers and royal marines carried out a forced march with heavy packs across the harsh, cold territory.
But all the truly daunting goals had been accomplished by the time the men reached their destination zones at the end of the march. In other words, the main difficulty the British faced was getting soldiers and marines in position to take on the Argentine soldiers in a set of infantry battles for control of the islands. At *that* point the odds were very much in their favour. The British soldiers were well-trained, well-equipped NATO troops, and they made short work of the poorly-trained conscripts that made up most of their Argentine opposition. Within a few days of the land battle beginning, Brigade General Mario Menéndez had surrendered the garrison in Stanley, on 14th June 1982.
Source: Kaitain Jones (Quora)
