@IPEdmonton there are more than two... #Akkoma and #GoToSocial are others, and there are mastodon "forks" like #Glitch and #Hometown that have different features too (my server is Hometown instead of plain Mastodon)
This came about because not everybody has the same tastes and needs such as:
Mastodon can be complicated to set up and consumes more resources. Some alternatives are easier to set up and cheaper to run
Sometimes there are geographic preferences due to origin of the software (#Misskey is from Japan, Mastodon is German)
Mastodon is written in Ruby and some people like to run servers in languages they are more familiar with (such as Elixir, Go, Python...)
Some people don't agree with Eugen's priorities or management style and want features or UI designs he doesn't want to implement like formatting posts or quote-toots so alternatives arise that do those things
Since they all interoperate not many people deliberately have multiple accounts just based on the platform...