It looks like a design choice. It started with a missing arm.

The diagonal seam on your baseball tee is called a raglan. The buttoned sweater in your closet is called a cardigan. Both take their names from British officers involved in the same military disaster. But the sleeve’s story starts with a missing arm. Lord Raglan lost his right arm at Waterloo. As the surgeon carried it away, he called him back. His wife’s ring was still on one of the fingers. According to the familiar origin story, his coats were cut to make dressing easier afterward. Instead of stopping at the shoulder, the sleeve extended to the collar, creating a diagonal seam from the neck to the armpit. Nearly forty years later, Raglan commanded the British Army in the Crimean War. One of his cavalry officers was the Earl of Cardigan, a wealthy aristocrat who reportedly spent around ten thousand pounds a year outfitting his regiment. While his men slept in the mud, Cardigan stayed aboard his private yacht. His immediate superior was Lord Lucan, his brother-in-law. The two despised each other. On October 25, 1854, Raglan watched Russian troops carrying away captured British cannons above Balaclava. From his position on a ridge, he could see what was happening. He sent an order for the cavalry to intervene. But down below, Lucan couldn’t see the guns Raglan meant. When he asked the messenger which guns to attack, the messenger allegedly gestured toward the far end of the valley. There, about a mile away, stood a different Russian artillery battery. Cardigan led roughly 670 horsemen toward it. They took fire from the front and both sides, reached the guns, and suffered further losses riding back. More than a hundred men were killed. When the survivors regrouped, only 195 still had horses. Back in Britain, Cardigan was celebrated for leading the charge. A knitted wool waistcoat, buttoned down the front and associated with what he wore against the Crimean cold, took his name. Raglan died before the war ended, but his name survived in the sleeve. The cut spread