The short answer is because that is the fashion of the day. The long answer can be found in the attached article. But essentially there is an etiquete for "high fashion" and the current trend for a 3 button suit is to button only 2. Which 2 depends on your frame. I am a heavier build so I button the lower 2, but more trim people tend to button the top 2 and leave the bottom open.
A relevant part of the article states: Double-breasted suits are almost always kept buttoned. When there is more than one to fasten (as in a traditional six-on-two arrangement), only the top one need be fastened; in some configurations, the wearer may elect to button only the bottom button, in order to present a longer line (a style popularised by the Prince George, Duke of Kent).
Today, single-breasted suits are buttoned while standing.[citation needed] This is because the current trend is for jackets which are loose. Prior to 1935 and again during the 1970s, men preferred fitted jackets which could be worn either buttoned or unbuttoned whether one was standing or not.[citation needed] In a three-button suit, all but the bottom button can be done up. Today, however, if only one button is done up, it is the middle one, if two it is the middle and top ones. This is because in general jackets today are made to be low-waisted. Prior to 1935, men preferred high-waisted coats and therefore the fashion was to button only the topmost button (if any was to be buttoned at all). In two button suits it is appropriate to button the top button, but never the bottom button only. If one is wearing a waistcoat, there is no need to button the jacket. The general rule in the Western World for the buttons (listed from top to bottom) is "Sometimes, Always, Never."
If one is wearing a four button suit, he may choose to wear the suit with two buttons done up (the middle two) or all of the top three. It is the fashion with a four button suit to keep it buttoned when sitting. The general rule in Western Dress with a four button suit (listed from top to bottom) is "Sometimes, Always, Sometimes, Never."
When a bespoke jacket has working buttons on the cuff, some men like to leave some of these undone to demonstrate that they can afford to wear a bespoke suit, but it is more proper to fasten all the buttons. A bespoke suit should be understated.
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