Because there can be. Because at some point somebody said I want to make a version thats like This and Does This. Or they said Fuck those guys making this version, theyre privacy invading scumbags. So they forked a version and made a new one. And that happened a couple times. Because it's a large community not led by a single person many different desires and needs arise. And since the code is free as in beer and as in speech anybody can fork some code and make it do whatever they need or want.

Why are there so many varieties of tomato?

Open source means lots of people making their own version of "best"

For example Unbuntu is really good for ease of use, Tails is really good for privacy. Others great for old computers without a lot of processing power, others great for coders and those that want to customize, etc.