To put it bluntly: the entire campaign was utterly ill-advised. It's not so much remarkable that it failed as that it didn't fail significantly worse.

The Dardanelles campaign (which is its proper name, since the campaign encompassed more than just Gallipoli) began with an exceptionally ill-advised attempt to force the eponymous waterway that divides what was then the Europe-based capital of the Ottoman Empire (Constantinople) from Anatolia and the rest of the Ottoman Empire's holdings.

The idea was simple: if the Entente could force the passage, they could significantly disrupt Ottoman military efforts (possibly even force a peace deal) and simultaneously link up with the Russians by way of the Black Sea. On paper, at least, it makes sense. The problem is that paper so rarely reflects the reality of warfare.

The initial attempt to force the passage was a complete disaster. The Ottomans had forts, mines, and ships of their own. The British and French lost three battleships and narrowly avoided losing two more (and a battlecruiser as well). The British probably should've known this wouldn't have worked, since they tried and failed when doing something similar in China during the Second Opium War, but the Entente had a tendency to underestimate the Ottoman Empire in WW1.

After the push failed, the Entente realized that they'd need to attack by land to knock the forts out of action before making another push. But since they figured they could force the passage even if they only controlled one bank, they focused all their efforts on the European side of the Dardanelles. The decision was reached to land troops at the very edge of the peninsula, possibly banking on the idea that they'd be able to seize a large bridgehead before the Turks could rush in enough troops to stall the landings.

The problem with the Entente plan at this point is that they had seriously underestimated the competence of Ottoman land forces. The quality of Arabic troops can probably be debated, but the Turkish troops that had recently undergo extensive training under the German system were significantly more competent than their counterparts in the Balkan Wars. The Ottoman commanders as well had seen significantly improved training. And the Ottoman commander on-site as the landings began was one Mustafa Kemal. There's a great deal of effort to portray him as some superb general... but he really wasn't. He was merely competent. And in the face of Entente incompetence, competence is itself remarkable.

Kemal, for all his mistakes (like the superbly foolish counterattacks in late April, 1915), correctly saw that the best way to address the Entente incursion was to scramble his troops (some of which were still filtering) and throw up a defensive line as near to the water as possible. Kemal also correctly anticipated where the Entente would land, which certainly helped (a significant portion of the Ottoman forces earmarked to repulse the Entente were on the wrong side of the Dardanelles when the landing began because von Sanders guessed wrong).

After the initial check on the Entente advance, things started to go poorly for them. The terrain was extremely rough, which meant that attacks (from either side) were often funneled into relatively narrow passes and promptly shot to pieces. At Anzac Cove, for instance, the Turkish counter-attack on April 25th (Anzac Day) was shot to pieces by all of two machine guns. But the Aussies were in no state to capitalize on repulsing the assault; they'd been losing troops all day to snipers and artillery fire, plus whatever they lost during the Turkish attack.

The Gallipoli campaign as a whole often looked like that. One side would attack, suffer losses, and pull back; the other side would attack, suffer losses, and pull back. Neither side would accomplish much beyond losing men. It was the Western Front in miniature.

As the front devolved into an effective stalemate, dysentery swept through both sides. Men quite literally became too sick to fight and the fighting capability of most battalions was significantly undermined. But while Gallipoli (and the Western Front) devolved into stalemates, the situation in the Balkans was dynamic. In October of 1915, Bulgaria joined the Central Powers. Bulgaria's entry into the war was the death knell for Serbian resistance: German, Austro-Hungarian, and Bulgarian armies carved the kingdom into pieces. With Serbia's demise came a now-unbroken line of communication from Berlin to Constantinople. Germany could now, almost at its leisure, send whatever men and materiel it wanted to support the Ottomans.

The Entente, apparently, realized in late 1915 that Serbia would actually need assistance if it was to survive the oncoming storm (because, apparently, the Entete was perfectly happy to let the Serbs struggle against the Austro-Hungarians and Germans on their own). The decision was made to dispatch troops to Salonika in Greece. These troops had to come from somewhere, some they came from Gallipoli. The French actually advanced into Serbia to cover the Serbian retreat, but were forced to withdraw in the face of Bulgarian numerical superiority. As attention was increasingly diverted from Gallipoli to the situation in the Balkans, and then resources and manpower, the decision was eventually made to simply abandon the campaign entirely. It hadn't worked, it had cost the Entente a staggering amount of men and materiel, and the Entente wasn't willing to continue supporting the offensive (which also bogged down a rather large portion of its naval assets in the Mediterranean).

It's also worth noting that the Salonika Front was ridiculously hypocritical. The Entente didn't have Greek approval to deploy troops in Greece -- they simply did it. Considering the British ostensibly went to war in defense of Belgian independence, their laissez faire approach to Greek sovereignty is certainly... interesting.

For a more detailed look into the Gallipoli campaign, I'd recommend Ottoman Army Effectiveness in World War I by Edward Erickson. It goes into rather deep detail on things like the von Sanders military mission, Kemal's actions in the opening weeks of the Gallipoli landings, and some of the myths and misconceptions that surround the Ottoman military in WWI.