There were two options for the Western Front in 1916: either attack in the Somme or in Flanders.
The Somme was chosen for a number of reasons. In December 1915 and through to January 1915, the French Commander General Joffre was advocating for an offensive in 1916 in the Somme region. He saw the Somme valley as a place where the British and French armies were physically touching and where their efforts could be most easily combined, and it had also been a quiet sector in 1915 and an offensive there might surprise the Germans. Still a meeting in January 1915 between Joffre and the British General Haig decided that the British would launch an attempt that summer to seize the Belgian coast, a diversionary attack in April in the Somme, while the French would launch a separate offensive on their own at an undetermined location sometime in June. A few days later Joffre changed his mind and asked for a British offensive focused on the Somme river sometime in the spring in addition to the operations already outlined. The British were understandably hesitant to commit to so many operations given the heavy casualties of the Western Front.
In February, King Albert of Belgium expressed his opinion that Belgian soil should be liberated by indirect rather than direct means (that is, no offensives that would devastate his already war torn country). After that, the British idea to launch an attack in Flanders was more or less rejected in favor of an offensive in the Somme as the French had outlined.
By mid-February, the Allies received word that a German attack at Verdun was likely and on 21 February their Verdun offensive was launched. The result, as you know, was that the British committed to a predominantly British-led attack in the Somme to relieve pressure against the French. Though the final plan would be debated in the coming months, Albert’s rejection and the German attack ultimately ended the chance for any other British operations in the summer of 1916.