Why didn't people redesign the key scheduling algorithm of DES to use longer keys instead of using 3-DES? To overcome short key length of DES, Triple DES was devised. What would go wrong or why didn't people redesign the key scheduling algorithm to use longer keys?
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.