Why does the Polish language not use the letters Č, Š, and Ž, and instead uses Cz, Sz, and Ż/Rz? Why has the Polish language never adopted the Czech convention to use caron/háček, despite being similar to Czech? Which are reasons for that?
"Tradition" is the best explanation. Czech, Polish, Slovak and even Hungarian face similar problems of having consonants lacking Latin equivalents. When the Latin alphabet was first introduced, there was a certain amount of chaos in dealing with the palatals. Originally, Czech used some of the spelling conventions also used in Polish – both languages randomly used digraphs as well as diacritics on s and z. Czech benefited from a work Orthographia bohemica published between 1406 and 1412, which eliminated the digraphs and trigraphs in favor of diacritics, eliminating rzss, zr, sr, rzs and so forth. This caught on in a number of other languages, but not as much in Polish, which also had an orthographic reform later at the end of the 18th century. (Unrelated) Hungarian underwent the opposite reform, converting (some) earlier diacritics to digraphs.