1) To encourage good security practices in the event of compromise or technical improvements. Original '90s "export approved" SSL certificates were only 56-bits. If sites still used those today, they could be easily cracked.

2) To guarantee a recurring revenue stream for TLS/SSL issuers. Originally certificates were $50 to $100/year and there was a big process around renewal and verification. I remember having to fax in corporate paperwork. What a pain!

I bet some guy with a ton of badges on his suit is asking the exact question in some Pentagon boardroom right now.

Since revocation is also a big pain.