I’ve not dug into this, but note that the expression “hardware store” appears in the 1850s; bumps up as the hardware store becomes a retail genre ca 1900-1915, and again in the 1940s (can’t guess why). Rises again in the 1970s as the hardware store genre/category shrinks and/or mutates… The rise in 1990s reflects nostalgia? appearance in nostalgic and romance fiction? More swan songs?
The earlier bump coincides with my own great grandfather’s entry into the business, chronicled here.

Will continue to ruminate. And drill down into the ngram report: what’s in the corpus? more scanned trade journals after 1900? Shift in terminology and even titles (Iron Age to Hardware Age, etc.)?

Haven’t managed to embed chart on this page, but here’s the link.

Looked again at this page today 28 August 2020, two years later. Did the same ngram search through 2019. Sharp drop!