By late fall of 1914, the flanks of the Allied and German armies were anchored along the Channel. Despite Germany's "fleet in waiting," the British were able to supply Allied forces via cross-channel shipping. Why, after the stalemate- especially, perhaps, after years of stalemate- did the Allies fail to attempt a large-scale amphibious invasion aimed behind German lines?
I recognize Gallipoli is a factor here. After the failure of that amphibious effort, garnering support for another one- both in Allied high commands and among the Allied public- would certainly be difficult. But would it have been more difficult than persuading the English and French to endure another Somme?
Was technology the issue? The development of the Higgins boat is considered by some historians to be an essential part of the D-Day operation, and certainly the Allies didn't have those in 1915. But neither had the Germans underwent construction of an Atlantic Wall in 1915.
This question has always nagged at me - can anyone answer it?
To add to the other great answer.
Jackie Fisher and Winston Churchill were OBSESSED with the idea of an amphibious operation to win the war.
The main options were the German North Sea Coast, or the Baltic Coast. However both required the High Seas Fleet to be handled in some way, which their massive shore defenses of mines and small craft made very difficult. Many of the specialty ships later used at Gallipoli(like the 18in gun armed monitors) were built with the shallow Baltic in mind.
The Baltic operation also would have been used to land a Russian Army, which later became an impossible option as well.
There were also plans to seize the German Flanders U-Boat bases or the island Heligoland in a lightning raid with light cruisers and destroyers running up with transports full of marines, with the Grand Fleet in the offing. Put the plans were considered foolhardy at best and suicidal most likely as there was little good intel on the shore defenses and the closer the fleets fought to Germany the more dangerous it was.
Gallipoli was in many was just a vent of their frustration that the navy was unable to be decisive in the North Sea, so they figured why not be decisive somewhere else? Fisher though had serious misgivings about a Naval only operation from the start and greatly preferred a joint operation, along with almost every naval officer.
After Jackie and Winston left the Admiralty amphibious operations on a grand scale were essentially abandoned for the rest of the war.
From Dreadnought to Scapa Flow: The History of the Royal Navy in WW1; Arthur Marder
The British DID have plans for amphibious operations on the Flanders coast, as part of the Third Ypres offensive in 1917. It was called Operation Hush, and would have seen a British corps land near nieupoort. However, Third Ypres came close, but not close enough, to breaking through the German lines, and so 'Hush' was never attempted. Small scale landings were made after the 5th Battle of Ypres in 1918, but by this point the Germans were being rolled up, and mobility had been restored to the battlefield.
The big issue here is, where would they attack? The German coastal defences were pretty strong, as the failed Zeebrugge Raid of 1918 demonstrated. Moreover, building up the necessary forces for such an operation would divert men and materiel away from the main front - the Western Front.
Besides, the Somme largely achieved it's goals: Verdun was definitively relieved, heavy losses (500 000+) were inflicted upon the enemy, who began construction of the Siegfried Stellung, necessitating the abandonment of substantial territory. Then, in 1917, the Arras offensive took Vimy ridge and made decent progress, 3rd Ypres (Passchendaele) greatly improved the British position around Ypres, while the Nivelle Offensive, although very costly and unable to live up to it's grandiose aims, laid the stage for further operations, like Malmaison, that would see the French push the Germans off the Chemin Des Dames ridge. Then there was the Cambrai raid: supported by hundreds of tanks, air superiority, predicted artillery, British infantry and cavalry breached the Hindenburg Line, albeit being pushed back days later.
Essentially, why go through the efforts off planning a complex operation, fraught with risk, requiring navy-army cooperation, to seize coastline, with the questionable prospects of somehow ending the war sooner, when the Allies could keep up the pressure on the continent, especially now that after the Somme, they were confident that they could, in fact, WIN THE WAR?!
Landing craft were developed for Hush, but they were in insufficient numbers for larger forces. Andrew Wiest has an essay about 'Hush' in Peter Liddle's "Passchendaele in Perspective".