The reason why kimono's are not very commonly worn anymore is because of numerous reasons;
a. There are not zips, buttons or drawstrings etc on a kimono. The design itself is incredibly simple (literally just a few rectangular bolts of fabric sewn together), so to wear a kimono properly and attractively it takes a lot of practise and skill.
b. Western clothes are cheaper- you need a lot of things to wear a kimono properly- undergarments like Juban/Nagajuban, Obi, tabi socks, lots of ties, a special board for under the obi to keep it flat, obijime, even a type of fake collar etc etc. Unless you're just getting a cheap cotton yukata set to go to a casual festival or hotspring in or something, when buying a full brand new kimono ensemble, the price can be very expensive (much more expensive than buying a new western outfit).
c. It takes more time to put on a kimono ensemble than to put on western clothes.
d. There are different types of kimono for every occasion. Many kimono's motifs are attuned with the seasons, months or even particular days of the year and wearing motifs out of season/month/day of kimono's for the wrong occasion or age (like houmongi is a very formal design kimono while a furisode is a young girl/womans kimono) is looked down upon a lot.
So western clothes tend to be a lot more flexible.
e. Kimono's aren't very comfortable to wear. You can move so much more freely in western clothes and so when you wear a kimono you have to change the way you move and even the way you sit. It can take some getting used to wearing a kimono. Western clothes are just so much more liberating to wear.
Kimono's can look absolutely beautiful, but there's so much time, effort & money that goes into wearing them, this is why most people don't wear them everyday in Japan anymore. Only certain professions still wear kimono on a daily basis (like
Sumo wrestlers & Geisha).