Tires / Yaris

An enquiry from one of our community members!

My wife has a Toyota Yaris which (in South Africa at least) apparently comes with some funny sized 15" wheels / tyres which cost around R1200 a tyre to replace (Thats the same as my BMW!)

Nobody can explain to me why we can't simply but a smaller wheel / rim / tyre on the Yaris to accommodate 14" tyres which cost about 1/3rd of this amount.

Can somebody possibly explain to me how this works and why this is the case?