Why is the connection between ancient Illyrians and modern Albanians difficult to prove through language?

The difficulty in proving a direct linguistic connection between ancient Illyrians and modern Albanians stems primarily from a severe lack of evidence regarding the Illyrian language.12

There are no surviving full sentences or phrases of the Illyrian language available for analysis; researchers have only three or four unambiguously identified lexical items, along with a limited number of personal names and place-names. Because of this extreme scarcity of data, historical linguists remain divided:1
* Some accept the presumption that Albanian is a direct descendant of Illyrian.13
* Others suggest Albanian may be descended from a close relative of Illyrian.1
* Many insist that the current state of evidence is simply inadequate to reach a definitive conclusion.1

While it is clear that Albanian is a distinct branch of the Indo-European language family, the lack of documentation for ancient Balkan languages makes it difficult to convincingly demonstrate its specific origins. Alternative theories, such as a derivation from the extinct Thracian language, also suffer from similarly sparse data, leaving the question unresolved.452