Why are there yom tov additions to the first b'racha before sh'mone esre in the morning and to the third, but never to the second?
(In maariv, by contrast, every birchas k'rias sh'ma has yom tov additions.)
I certainly don't know "why," but the flow of many piyyutim goes Yotzer-Ofan-Zulat.
But not all! See, for example, the piyyutim for Shabbat Chol Hamoed Pesach, where the flow is Yotzer-Ofan-Meorah-Ahavah-Zulat-Geulah. (These piyyutim are in the Rinnat Yisrael machzor, for example, but the ArtScroll machzor doesn't have the Meorah or Ahavah, with a note in the commentary to the effect that the composition is missing lines.)
There are also piyyutim for special shabbatot that include an Ahavah, such as Shlach, Va'etchanan, and Eikev (describing the mitzvot of shma, tfillin, and tzitzit) or the shabbatot between Pesach and Shavuot (about the relationship between G-d and Israel). I have a siddur at home with these piyyutim, which is why I'm aware of them. Unfortunately, I don't know how to find links online to them.
Not really an answer, but perhaps once it was seen that the first bracha is flexible enough to change once a week (it has weekday and Shabbat versions), later liturgists had license to make other additions to it. (I assume that HaKol Yoducha and Kel Adon predate the other "yotzros", is that correct? Are the former attributed to the Men of the Great Assembly?)