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Why did Internet security systems move from 40-bit keys to 128-bit keys?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

Internet security systems transitioned from 40-bit keys to 128-bit keys primarily because advancements in computing power rendered 40-bit systems insecure against brute-force attacks.12

A 40-bit key provides a key space of $2^{40}$ possibilities, which is approximately $10^{12}$ combinations. By the end of the 20th century, the rapid increase in the processing speed of personal computers allowed these systems to test the entire key space in a matter of hours. By distributing this task across multiple machines, the time required to break such a key could be reduced to mere minutes or seconds.1

Because the security of a cryptosystem is fundamentally tied to its work function—the computational effort required to exhaustively search the key space—the 40-bit standard became insufficient for protecting sensitive information in the growing era of e-commerce and Internet communications. The shift to 128-bit keys provided "strong encryption," significantly increasing the work factor to a level that protects transactions against modern computational threats.12