Why does apt install, install an older version of Redis?

Question

I am learning Ubuntu and I am installing Redis. When I use the following guide, an older version of Redis is installed. 4.0.x

https://www.digitalocean.com/community/tutorials/how-to-install-and-secure-redis-on-ubuntu-18-04

According to redis.io, the latest stable version is 5.0.5. Why does apt install install a 4.0.x version and not the latest 5.0.5 version?

I'm using Ubuntu 18.04.

Thanks!