Why is the Tsugaru dialect so hard for native Japanese speakers to understand?
The Tsugaru-Ryukyu region, where Tsugaru-juku lies, has a number of distinct language features that make it difficult for speakers from elsewhere in Japan to learn. Some of these are:
1) The lack of any phonological changes or sound simplifications that other parts of Japan have undergone over time due to their relative isolation. 
2) The heavy influence of loanwords from western languages (English, French, etc.). This means words don't change too much even if they should logically fit with surrounding dialects better than some local word does. In fact this may cause issues for those trying to read the original writing rather than hearing spoken dialect.  3) There being two sets of verbs which mean essentially the same thing but one is more polite. It can be quite challenging when talking to someone whose speech patterns and grammar you aren't familiar with as how best to reply to something.
4) Lack of stress markers, which makes learning about word order harder compared to say Kyoto/Tokyo where such information will tend to cluster at particular positions based on the verb conjugation system.
5) Vocabulary is heavily influenced by Shinto and Buddhist religious terminology - this might seem strange because Shinto and Buddhism didn't spread all that far beyond Japan proper, but the religion tends to prefer abstract terms over common ones found across different religions like'mother' versus things related specifically to females, such as wife, sister or daughter. So if I am right, many Japanese would consider me incorrect were I not calling God Mother rather than using Father instead since we share both father's mother!