Short answer: After WW2.

Long answer:

During the Meiji Restoration, Japan ended its isolationist policy and hurried to modernize and catch up with the rest of the world. As a result, Japan was exposed to all sorts of Western elements of culture, including fashion, and wearing Western clothing was seen as modern and forward-thinking, while kimono was backwards and a feature of the past feudal era.

Around the Taisho Era, many people started shifting towards Western clothing. Again, Western clothing was seen as fashionable and far more practical than the stiff, encumbering, and old kimono. For women, sewing a simple skirt at home was easy to do compared to the numerous panels required to sew a kimono together.

As well, even other elements of Western culture changed the way people dressed. For example, kimono lends itself well to kneeling gracefully on tatami floors. As Japan modernized, people started to sit on chairs at tables, which makes it a little more awkward when wearing a kimono with the obi knot on one's back. In fact, one type of kimono was born from this new seating. On most formal women's kimono, designs are only present below the waist, but if one wore such a kimono while sitting at a table, the designs would not be seen at all. The houmongi - a semi-formal kimono with designs above and below the waist - was then created.

Then, WW2 hit. With resources being so scarce, any luxury was frowned upon, including kimono. People - even the traditionalists - shifted to simple work clothes that were easy to move around in, while kimono were kept in storage, ripped apart for the fabric, or sold for food or other necessities. This was basically the death knell for everyday kimono.

Kimono essentially lost out to Western clothing as everyday wear for the above reasons after WW2. The exorbitant amount of money required to buy a kimono, obi, and all the related accessories did not help, either.

Around the 1950 and 1960's, department stores began to market kimono as special clothing to be worn for celebratory occasions. And ironically enough, those who wanted to promote kimono did so by (unintentionally) killing it. As people forgot how to put on kimono, schools for dressing in kimono popped up, introducing numerous rules surrounding kimono - hems had to be a certain amount of centimeters above the floor, collars had to be crossed at a particular angle, padding had to be placed around waists and backs to create a perfect cylindrical shape, etc. Dressing oneself in kimono became a daunting task full of strict rules. Even now, there is a myth that it is impossible to dress yourself in kimono without help because of how difficult it supposedly is (it's difficult for sure, but not impossible). Contrast images of women wearing kimono today compared to women wearing everyday kimono before WW2. You'll see pre-WW2 women wearing their obi crooked or their ohashori uneven because such rules demanding perfection simply did not exist.

So basically, a combination of efficiency, economy, and comfort spelled the end of kimono as everyday wear, with the final nail in the coffin being WW2.