Why didn't people redesign the key scheduling algorithm of DES to use longer keys instead of using 3-DES?
Same as the reason of why not switching to AES (or any other 80+ key bits algorithms available at the time). 

Because we want to reuse whatever "hardware" (chip design) resources available to do the blockcipher calculation, and re-designing the key schedule would prevent us from doing so since the existing hardware cannot compute the new key schedule.