Oh, there are a few different schools of thought on this one, but I'll pitch in with two of the main theories.

Ernest Gellner supposed that it was the very nature of industrialisation that created nationalism.

His theory focused on the political and cultural aspects of that transition. In particular, he focused on the unifying and culturally homogenizing roles of the educational systems, national labor markets, and improved communication and mobility (in the context of urbanization).

https://en.wikipedia.org/wiki/Gellner%27s_theory_of_nationalism

Gellner saw sees elites imposition of high culture on general society through the likes of education and labour markets as the main reason nationalism spread.

To link further to your question, industrialisation in this case was a result and a pre-requisite for nationalism. The need for workers, skilled ones at that, allowed for state created entities controlled by elites to push their agenda and homogenize in the name of industrial progress.

Another theory that is quite a popular one comes from Benedict Anderson who has more of a marxist take on the advent of nationalism. Anderson argues that language and print media were the biggest motivators behind nationalism. He sees the spread of newspapers and printed word creating and "imagined community" where people begin to share ideas. Print was originally restricted to the elite classes in Europe (and in other countries) but once books started to be written in the vernacular (instead of Latin) people of a certain language group started to have a sense of community. The stories shared among a community in this case helped create the ground of a shared experience and perhaps aided in the belief in common creation myths of certain nations.

Anderson also points to movements to abolish monarchical rule defined by divine right and print capitalism as key reasons to the spread.

This is really a diverse topic and I really do recommend you reading Anderson's "Imagined Communities" and Gellner's "Nations and Nationalism" as good starting points on nationalism theory.