The answer is simple here.
Only 22 shrutis (places for remaining steady or nyasa) exist naturally on a single string.
As we know, there are 12 identifiable musical notes in a Saptak (Octave), namely S, r, R, g, G, M, m, P, d, D, n, and N. What is surprising is that out of these, Shadja and Pancham are playable at 1 precise point each on any string; and each of the other 10 are actually spread over a 'region' on the string !
For e.g. if we pluck a single string, it first makes the sound of "Shadja" or "Fundamental tone". As we go on moving the point of playing the string from any side, it makes some unidentifiable sounds till we reach the region of r.
This is the lowermost point of perception as r by human ear (let us call it as r1). Continue ahead, and soon we reach another point which is the highermost point of perception for r by human ear (r2). In between these 2 points is the 'region' of r as it exists naturally on a string.
Thus, for each of the 10 notes other than the Shadja and Panchama there are lower and the higher 'points of perception by the human ear', creating a region on the string !
When we start playing from one end, (see figure below)
The frequency starts increasing from "Shadja", and a point is reached where we can perceive the note as "Komal Rishabha" (See r1 : producing frequency of 105.35). We can move further and still the perception of Komal Rishabha continues.
This happens till we reach r2 (producing frequency of 106.666666). Beyond r2, the string produces a non-recognisable (besur) sound. This region can be called as 'No man's region'(-------*------).
Thus, there are 2 limits for Komal Rishabha, r1 (lower), and r2 (higher), creating the "Swarakshetra" for Komal Rishabha (------*-----).
Go further and R1 (111.111111) is played at 90 % and this is the lowermost hearing limit for the perception of "Shuddha Rishabha". Similarly, R2 (112.5) is played at 88.88 % and this is the highermost hearing limit for the perception of "Shuddha Rishabha".
Only if we pluck the string anywhere in between 88.88 %, and 90 %, human brain will perceive the note as "Shuddha Rishabha" ("Swarakshetra" for Shuddha Rishabha -----*-----).
We find 2 such points each for 10 notes : r, R, g, G, M, m, d, D, n, and N; a total of 20 points. For Shadja and Pancham, there is only 1 point each. This makes a total of 20 + 2 = 22 shrutis.
All the frequencies 'in-between' the 22 shrutis (those in 'No man's region') are called as "Nadas' and they are used to 'connect' the shrutis for creating 'alankars' (e.g., aalap, meend) in an actual performance.
However, we can not 'stay' on them as they would sound 'besur' (out of tune). In other words, Nadas (in No man's region') are connections, and 'Shrutis' are stations.
The 12 positions of the 12-Tone-Equi-Tempered (12-TET) scale lie exactly 'in-between' the 20 points for each of the 10 notes, and thus, lie at un-natural positions.
See Topic 33 : Practical Use of Ratios : Playing all the 22 Shrutis on any String.
Finally what is amazing is that these 22 points on any string produce frequencies (shrutis) which arise only from the natural triad of Shadja:Gandhar:Pancham as explained in Topic 22. None of the other points on the string have any relation to Shadja:Gandhar:Pancham thus proving that only these 22 shrutis are natural.