Why did William Herschel make his own telescope mirrors?

William Herschel began crafting his own telescope mirrors primarily because he was determined to study distant celestial bodies, which required instruments with larger mirrors than those available from commercial opticians at a reasonable cost.1

Driven by a desire to observe beyond the Sun, Moon, and planets, Herschel realized that larger mirrors were essential to collect sufficient light for his research. When he found that the capacities of local foundries could not meet his ambitious requirements, he took on the task himself, grinding mirrors from metal disks composed of copper, tin, and antimony. His persistence eventually led to the production of mirrors of such superb quality that his telescopes surpassed those used at the Greenwich Observatory.1