Before beginning I think it's important to say that the idea of ascension to kami did not stem from Kokka (State) Shinto, and instead has been a common thread in Shinto thought throughout much, if not all, of its history, despite it being a largely decentralized belief system for much of that time. Although alternate angles exist (such as traveling to Yomi-no-Kuni, an underworld, or Tokoyo-no-Kuni/Ne-no-Kuni, the land of root/land across the waves) these are largely considered to be lesser realms in the grand scheme of the 'spirit world,' and hence fall as subsets to the ascension model.

I think it's also crucial to define kami in a broader sense. Kami means, "an existence that invokes awe." It does not translate directly into "god" or "spirit" but rather encapsulates these terms and so much more. In Shinto, the sun deity Amaterasu-Omikami, a particularly wondrous stone or tree, and one's great-grandparents all fall under this same umbrella of kami. There is no theological or metaphysical distinction between them. 

With that said, the short answer is that becoming kami was already widely accepted. The greatest victory of the Meiji Restoration and into Pacific War-era Japan theologically was connecting that belief directly to the Japanese State.

In a longer response... The concepts of chinju no kami (kami of local area), ubusunagami (kami of your hometown), and ujigami (clan protector kami) predate usage of the term Shinto itself. I think for this conversation it's important to remember that ujigami specifically are traditionally one's clan or familial ancestors elevated to kami after death. There was already a precedent in Japan for one's deceased to become kami. This type of thing became very important in wars fought within Japan's borders, as ujigami would protect soldiers in battle. It was assumed that when one died, they would live on as ujigami, watching over their family or clan and providing divine intervention where possible. 

As Japan entered the international stage (and even beforehand as the nation unified), a necessity grew for this concept to expand to include the entire state. The philosophy of 国体 (kokutai, often translated as national polity) grew out of the Meiji Restoration and, among many other things, provided exactly this. It positioned the Emperor and his empire as the head of the national clan. When one died in service to the Emperor, not only was it the highest honor they could provide the empire, but also themselves. Additionally, death in of itself isn't much to be feared (equal parts thanks to Buddhism and Shinto, I personally feel), and one will live on as ujigami anyways, serving their own way beyond death to protect and serve the empire. 

This is the philosophy and policy that carried into the Pacific War. The unification of church and state, so to speak, was a massive part of the indoctrination of the Japanese people, beginning in the 1870s and carrying up until the end of the war. After the surrender, Kokka Shinto was disbanded and the connection to the state was removed. It's important to note throughout this that while many threads carried through before and after, State Shinto is almost universally considered a bastardization of faith by authoritarian government and not true to the spirit of Shinto whatsoever. I hope this helps!