In 1942, the answer would have been, "because Britain didn't have enough troops." They were then weak "everywhere" along the line.
But by early 1944, after two years of the Burma Campaign, Britain and its allies (America and China) had an overall superiority of troops and equipment on the Burma front. Why, then, did Britain allow Japan to have a local superiority in the middle of the line while the main British forces were concentrated further south, opposite Arakan, Burma, and American and Chinese forces at the north end of the line, aiming for Southwest China.
Was General Slim employing a "Cannae" like strategy of luring in the Japanese into a trap in the center, and then closing in on the flanks? Elements of this actually occurred during the campaign.
Did the British somehow underestimate local Japanese strength in the center of the line, despite their overall superiority?
Did the British, for "imperial" reasons give greater priority to recovering Burma than to protecting India and the "Over the Hump" air link to China further north?