With ipv4 there are only 4 billion addresses or so. That might sound like a lot but we really chewed through them a while ago. We came up with a way to kind of fake more with NAT where one address can map to many machines but it was a bandaid.

Ipv6 can handle some 340 undecillion addresses if I'm not mistaken. Not gonna run out any time soon.

There's a lot of extra features baked into how ipv6 works that make things more efficient, potentially faster, and easy to maintain but those are all secondary benefits. They're great, but not the main reason for the switch.