alt Hacker NewsLets Encrypt are doing is because of the decision that CAs and browser makers made that it needs to be reduced (browsers have been reducing the length of certs that they trust).
The why is because it's safer: it reduces the validity period of private keys that could be used in a MITM attack if they're leaked. It also encourages automation of cert renewal which is also more secure. It also makes responding to incidents at certificate authorities more practical.
> it reduces the validity period of private keys that could be used in a MITM attack if they're leaked
If a private key is leaked, 45 days is sufficient to clean-out the accounts of all that company's customers. It might as well be 10 years.
If cert compromise is really common enough to require a response then the cert lifetime should be measured in minutes.