Answer:
1. Weight.
2. Large lens will tend to deflect under its own weight
Explanation:
Hello,
Weight is a big part of it. There’s a reason the largest working refractor on Earth (the Clark refractor at Yerkes Observatory in Wisconsin) has a 102-cm objective; a 125-cm lens was created for the Paris Exposition of 1900, but the accompanying telescope was a pain to use (very hard to aim) and was scrapped when no one wanted to buy it.
Lens can only be supported at its edges unlike mirrors, and a large lens will tend to deflect under its own weight unless it’s so thick that it won’t transmit much light. It is also extremely difficult to cast and polish a glass blank of such huge size, which is why (see List of largest optical refracting telescopes - Wikipedia) almost no one has tried building one in over a century.
Best regards.
