halacha - Why is Shmita only Rabbinic nowadays?

I was reading up on Heter Iska and it said that Shmita and Shmitat Kesafim are only Rabbinic today.

Why do the Biblical laws no longer apply?

Answer

Rambam Shemitta veYovel 10:10,12

משגלה שבט ראובן ושבט גד וחצי שבט מנשה, בטלו היובלות--שנאמר "וקראתם דרור בארץ, לכל יושביה", בזמן שכל יושביה עליה: והוא שלא יהיו מעורבין שבט בשבט, אלא כולן יושבים כתקנן.‏
...
ובזמן שאין היובל נוהג, אין נוהג עבד עברי, ולא בתי ערי חומה, ולא שדה אחוזה, ולא שדה חרמים, ואין מקבלין גר תושב; ונוהגת שביעית בארץ מדבריהם, וכן השמטת כספים בכל מקום מדבריהם כמו שביארנו.‏

Once the tribe of Reuben and the tribe of Gad and half the tribe of Menashe were exiled, the Jubilee ended, as it says "and proclaim liberty throughout the land unto all the inhabitants thereof" when all its inhabitants are on it, and they cannot be mixed up tribes with tribes, rather each sitting as appointed [in their tribal plots].
...
And when the Jubilee is not practiced, neither is [the laws of] the Jewish slave, nor the houses of the walled cities, nor the inherited land, nor the consecrated land, nor do we accept Gerei Toshav. And Shemitta is in force in Israel by rabbinic decree as is the nullification of loans in all places by rabbinic decree as we explained. (my translation)

Essentially, Shemitta doesn't apply when Yovel doesn't, and Yovel doesn't unless the Jews live in Israel by tribe.