This type of distribution is actually a common thing and is an example of the Pareto principle in action (or the 80-20 rule).

Quoting from Wikipedia[1]: "Pareto noticed that approximately 80% of Italy's land was owned by 20% of the population."

This phenomenon happens in various fields and in the case of Tatoeba this would translate into "80% of the sentences are contributed by 20% of the users".

I never invested time into figuring out what stops the users from contributing (that would be a very interesting research to do though), but my guess is that aside of spammers and bots, it's one of these reasons:
- website is not intuitive enough and people can't figure out how it works
- despite some initial good motivation, people ended up not being inspired enough to create sentences or not confident enough to translate
- or perhaps they tried to contribute but coincidentally, the sentences they wanted to add were already added, or they couldn't find anything easy enough to translate because all the easier sentences were already translated into their language
- procrastination: they register today then tell themselves they'll contribute tomorrow, but in the end never find the time to
- just curious: they just want to see what's available to them when registered
- by reflex: so many websites ask for registration nowadays, I wouldn't be surprised if some people just registered without really thinking
- to "reserve" their username: someone you stumble a website that you find kind of nice and you want to make sure your username is not taken by someone else (I happened to have done that myself on some websites when I was younger)

And there's probably hundreds of other reasons that my imagination cannot comprehend.

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[1] https://en.wikipedia.org/wiki/Pareto_principle