What criteria is used by THE to determine whether a university gets a WUR ranking or not?
The THE gives a reasonable breakdown of how institutions are rated (based on 13 measurables) and which countries they accept institutions from - based on European Higher Education Area and OECD criteria.

Here is a link to the detail they give : https://www.timeshighereducation.com/world-university-rankings/europe-teaching-rankings-2018-methodology

And here are the questions they use (added to prevent link-rot):

These are the survey questions that are used in the ranking (students were asked to answer on a scale of zero to 10):

To what extent do you have the opportunity to interact with the faculty and teachers at your university as part of your learning experience?
To what extent does your university provide opportunities for collaborative learning?
To what extent does the teaching at your university support critical thinking?
To what extent does the teaching at your university support reflection upon, or making connections among, things you have learned?
To what extent does the teaching at your university support applying your learning to the real world?
To what extent have the classes you have taken in your university so far challenged you?
If a friend or family member were considering going to university, based on your experience, how likely or unlikely are you to recommend your university to them?
Do you think your university is effective in helping you to secure valuable internships that prepare you for your chosen career?
To what extent are you satisfied with the access to learning materials (such as library, texts, labs and online material) at your university?
To what extent are you satisfied with the quality of the learning environment (such as lecture halls, study areas, and labs) at your university?
To what extent do you think that the teaching at your university is supporting you to learn skills that will be useful in the world of work after university (for example writing, numeric, presentation or other skills)?
To what extent do you have the opportunity to learn about and make connections to the world of work after university (for example through work placements, careers advice)?

Exclusions

The ranking only includes universities in countries that are a full member of both the European Higher Education Area and the Organisation for Economic Cooperation and Development, and that are classified as a “developed economy” by the United Nations.

Universities are eligible for inclusion only if they offer courses and programmes that lead to officially recognised higher education degrees such as bachelor’s degrees and master’s degrees or equivalent; are focused on more than one narrow subject area; and have at least 5,000 enrolled students at bachelor’s or equivalent level. Institutions must also receive a minimum of 50 valid responses in the THE European Student Survey to be included.

In cases where institutions have not provided the required data, we have used data from other external sources or provided an estimate. We have excluded institutions if this has not been possible or if the data for a particular institution are too sparse. We also excluded countries for which too few institutions submitted data, resulting in a lack of comparison with institutions within the country to create a meaningful score.


Then further reading may be interesting here : http://theconversation.com/rethinking-university-rankings-we-need-to-talk-about-quality-and-inequality-of-teaching-104071