Jeans didn't exactly replace anything - it's not that there was one ubiquitous type of pants that tapped out in favor of denim.

Denim is simply a cotton twill, colored after weaving with indigo or a synthetic imitation. Cotton twill is pretty ordinary, historically, as a strong yet supple fabric: it was the fabric typically used for corsets in the nineteenth century, as well as other items, like trousers, that might be under a lot of stress and see a lot of wear - at the time, "jean" was specifically a somewhat delicate twill, while denim was definitely intended for labor. The use of indigo makes it interesting, as raw indigo requires a chemical bath (in the past, it often involved urine) in order to break it down into something that will attach to a textile, in a process much more complicated than the ones used for other vegetable-based dyes.

Levi Strauss, a Bavarian immigrant, came to New York City in 1847 with his natal family, which together ran a dry goods business. (Dry goods are basically "stuff that's not groceries" - often involving textiles.) In 1852, he was sent as an emissary to San Francisco, to set up another branch of the family business to cater to the increased population of California following the Gold Rush, and he set up his store in 1853, selling ready-made garments, bolts of fabric, accessories, handkerchiefs, wallpapers, and the like, to miners and the many, many other people who made up the city. He was actually supplying fabric to the inventor of blue jeans, Jacob Davis (another immigrant, but from Latvia), a tailor in Reno: Davis was asked by a customer in 1870 to create a pair of hard-wearing pants for her husband, and he used cotton duck canvas reinforced with rivets on the seams, as was done with horse blankets at the time. As this was an effective way of strengthening the trousers, particularly at the pockets, people who saw these in action wanted their own, and within a few years Davis knew he needed to patent this method. He chose to ask Strauss to be his business partner, and both of their names appear on the patent in 1873. Davis was by this point using a denim sourced from a manufacturer in New Hampshire that referred to that particular line of cloth as "XX", which is what the original Strauss & Co. denim jeans were then named. (Duck continued to be part of the plan until 1911, when the company chose to only work with denim.)

The riveted denim trousers were very popular, but only with manual laborers - because during the late nineteenth and early twentieth centuries, there wasn't really a strong perception that working-class dress or streetwear had more "authenticity" or coolness. These manual laborers included cowboys, however, and when dude ranches, catering to eastern tourists who wanted to experience the lifestyle for a few weeks, opened in the 1930s, Levi Strauss & Co. started a line called "Dude Ranch Duds" that were aimed at the middle/upper-middle/upper class consumer who wanted the style without needing anything quite so hard-wearing, possibly not even on an actual dude ranch.

During and shortly before World War II, casual clothing became a kind of youth uniform, particularly on college campuses. I've written about that before, so I'll just summarize it: students started to rebel against dress codes by wearing "sloppy" clothes (which could include jeans), pushing back until the codes were gradually worn down to next to nothing. Post-WWII, this seems to have become a real anxiety, with hoodlums and delinquents in movies being pictured in rough clothes - including jeans! Authority figures like the police and educators railed against it as a sign of bad behavior, which drove its popularity sky high, of course, and retailers were happy to meet the demand. At the same time, the association of (lighter) denim with sportswear was strong enough that even ordinary families were wearing new Strauss & Co. lines like "Denim Family" in their off hours. By the late 1960s, everyone was wearing them.

A very detailed source on this is "Blue Denim by the Bay: The Levi Strauss & Co. Archives", by Lynn Downey, in Costume (2009, vol. 43).

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