(The reason was probably because it is easier to remember Shoin-zukuri style for the Heian period, Buke-zukuri style for the Kamakura period, Shoin-zukuri style (a traditional Japanese style of residential architecture that includes a tokonoma) for the Muromachi period and Sukiya-zukuri style (built in the style of a tea-ceremony house) for the Edo period.
（おそらく平安時代-寝殿造、鎌倉時代-武家造、室町時代-書院造、江戸時代-数寄屋造、というのが覚えやすいためと考えられる。