Why do we tune to A=440?

I found this fascinating bit of information on page 205 of the March 1918 issue of The Etude under the heading "Musical Questions Answered."

Q. Why is there still some dispute about pitch? We all supposed that the matter was settled by the adoption of "International" pitch (A=435), but now we have a good deal about an A-440.

A. The pitch of A=435 was fixed in Paris for a temperature of about 60 degrees, but in America and also England it is customary to have the theatres and concert halls considerably warmer (say about 70 degrees), and wind instruments rise in pitch with a rise of temperature. The oboe is commonly used to give the pitch to a large orchestra, and the same oboe which would give A=435 at 60 degrees would give A=440 at 70 degrees, approximately. Theoretically the expansion of the material of a wind instrument when heated would lower the tone by increasing its size, but the effect is so slight as to be negligible, whereas the effect of a different temperature of the surrounding air is very plainly noticeable.

Here is some more pitch-related information.