Is Lithuanian so conservative because it spent all these centuries developing additional cases from Uralic influence and then losing them again?Nouns and other parts of nominal morphology are declined in seven cases: nominative, genitive, dative, accusative, instrumental, locative (inessive), and vocative. In older Lithuanian texts, three additional varieties of the locative case are found: illative, adessive and allative. The most common are the illative, which is still used, mostly in spoken language, and the allative, which survives in the standard language in some idiomatic usages. The adessive is nearly extinct. These additional cases are probably due to the influence of Uralic languages, with which Baltic languages have had a longstanding contact. (Uralic languages possess a great variety of noun cases, a number of which are specialised locative cases.)
(Sorry, this is the level at which I understand IE linguistics.)