Given that we continuously increase our joy until Av and continuously decrease it until Adar, why don't we say that from after Shushan Purim on we decrease our joy and from the tenth of Av on we increase it, such that Purim is the happiest day and Tishah B'Av is the saddest?
Probably because that would be too much joy for Purim, given that gezerah shavah that connects Purim to Yom Kippur:
יליף פורים פורים מיום הכפורים
We can be happy on Purim, but in the spirit of Yom Kippur, we can't allow it to be the happiest day of the year.
To appreciate the contrast of the "happiest" and "saddest" day each year, I think you'd have to assume that each year has an equal time interval between these two periods.
The interval from after Shushan Purim until the 9th of Av is the same each year. However, the interval from the 10th of Av until the following Purim is not the same each year. Cheshvan and Kislev have varying lengths and there may be a leap year. So the "happy" interval is unequal.
There was probably some fear among the sages that people might say, "Well, we were happier last year than this year (because last year was a leap year)." Then, when Purim came, they would be less happy on a non-leap year than on a leap year. We want people to arrive at Purim with the same amount of happiness.