Because it was shot on film, that's why we've been able to get really high-quality restorations of old episodes in recent years.
And he was hilarious in the show!
I just checked his Wiki entry and was again amazed at his multi-faceted career. Thanks to his innovations, I Love Lucy has stayed in the public eye while so many badly filmed situation comedies - that I loved as a kid - have faded into oblivion.
Back in the day - a TV show was broadcast and then GONE - off into the aether - never to be seen again. The idea was “who would want to see a TV show after it has already aired?”.
Desi recognized that they were producing comedy GOLD and it was too good for just a one time showing. He asked the studio to film it and they said No. He asked if he could pay to have it filmed and then he would own the rights - and they said “sure - but who will ever want to watch it after it has already aired?”
Answer: Generations untold.
Happy Birthday Lucy!