Rabbi Sorotzkin in Oznayim Latorah explains that there are two reasons why Rachel had a monument. The first is that since Yaakov changed the name of her son (Ben Oni to Binyamin) he put up the monument to show that it was not because of any lack in her. He also pointds out that she was buried "by the side of the road" and a monument was necessary so that the family (and her descendants) would be able to identify the location and be able to pray there.
Rabbi Munk also points out that the Jews being sent into exile prayed there for mercy.
Devorah was buried at an easily identifiable spot (as explained by Rabbi Sorotzkin in 35:8). As a result she did not need a different monument.
ח וַתָּמָת דְּבֹרָה מֵינֶקֶת רִבְקָה וַתִּקָּבֵר מִתַּחַת לְבֵית אֵל
תַּחַת הָאַלּוֹן וַיִּקְרָא שְׁמוֹ אַלּוֹן בָּכוּת:
Rav Hirsch also translates this pasuk as
8 And Devorah, Rivkah's wet-nurse died, and she was buried under the
oak; and he called its name Oak of Weeping.
Note that Chabad.org translates this as
8 And Deborah, Rebecca's nurse, died, and she was buried beneath Beth
el, beneath the plain; so he named it Allon Bachuth.
Which could mean that it was still identifiable or that it was marked as a burial spot (for the purposes of tum'ah and taharah) and did not need a special monument (as the place itself was still identified by name). However, there is no explanation offered and this is just my guess as to what it means (given this translation).
