Perhaps because each of the dates commemorate multiple things.
Five Tragedies happened on the 17th of Tammuz:
Moshe broke the Tablets
The Tamid offering was interrupted
A Sefer Torah was burned
An idol was placed in the Beit Hamikdash
The walls of Jerusalem were breached during the second temple
The 9th of Av has always been a day of tragedy, and many tragic things have happened on that day. Some of them are:
The Jews in the desert cry over the report of the spies, and G-d decrees they would die in the desert for 40 years. For the next 40 years some of the Jews would die on that day.
Both Holy Temples destroyed
Bar Kochba rebellion crushed at Betar
Expulsion from Spain.
and many more.
If you read the slichos of the day, you can see that we fast on the 10th of Tevet for three things that happened on three consecutive days (although not in the same year):
8th of Tevet: The translation of the Torah into Greek, an occasion the Rabbis compared to the making of the Golden Calf.
9th of Tevet: Ezra Hasofer passed away
10th of Tevet: Siege of Jerusalem began
On the other hand, Tzom Gedalia only commemorates one thing, the assassination of Gedaliah ben Achikam and the dispersal of remaining Jews in Israel.
The 15th day of Av is also celebrated for many reasons, as enumerated in the Gemara (Taanis 26B).
The only one I'm not sure about is the 15th of Shvat, and that may be because there is a disagreement in the Talmud (Rosh Hashana 2A) between Hillel and Shamai about when the new year for trees is. So in order to show that we rule like Hillel, we call it Tu B'shvat.