Why do problems such as Travelling Salesperson 2D have 0 full solves?

For problems where you are not assigned a score, "Full solves" counts the number of authors that have at least one submission receiving an accepted verdict. For problems where the problem author has specified a maximum score, it counts the number of authors that have at least one submission that gets this score (or more). 

Some problems, such as Travelling Salesperson 2D are designed in such a way that it's tremendously difficult to achieve the maximum score of 50. If a user were to fully solve it by getting a score of 50, they would be considered to have fully solved it. 

Finally, there are also some problems where the author has not specified a maximum score, since they are not aware of any reasonable upper bound. In these cases, you will never be able to fully solve it.