Because it solves real problems (it's very easy to use, and I use a lot). It works similar with HD wallet in bitcoin or ethereum. In HD wallet, you can derive accounts for many crypto currencies from one seed password which is what master password in lesspass does.

The counters in lesspass, can be used to derive another password for the same website, if one want to change the password for the website.

HD wallet also has the similar functionality. If you want to change the bitcoin address, just derive another account from the seed password.

And the solution to keep the seed password of HD wallet safe can also work with lesspass. such as [OpenSK][1].

[1]:https://github.com/google/OpenSK