There is a big problem when you learn hilchos shemitta, there is no real mesora of pask. In every other area of halacha there is a mesora of psak going way back. Where I live there are constantly pamphlets circulating which state that this is wrong and that is wrong etc. These are usually just ignored (even if the person has good arguments) because we know that we pasken x. 2 simple examples:
1. The heter for Chadash in Chutz Laaretz is very weak, based on all principles of psak we should be machmir, and yet, the majority of Jews in Chutz Laaretz rely on the heter.
2. The idea that a Reshus Harabim needs 600,000 people has no real source in the Gemara and is a minority opinion in the Rishonim. Yet, just about every eruv relies on this shitta and the majority of the frum population uses eruvin.
These kinds of things are only possible because we have a mesora of psak in these areas.
The truth is, this is being eroded as Dr. Grach in his famous article Rupture and Reconstruction pointed out, there is a big movement back to the sources and away from mesora and a lot of chumras have come into the mainstream because of this.
By hilchos shemitta, there is basically no mesora. Until the 1880's, no one had really encountered these issues since the destruction of the Beis Hamikdash and therefore it is very difficult to know how to pasken. In the few places that there is a mesora, the psak is clearer, for example, whether fruits grown by a non-Jew have kedushas sheviis we have a clear psak from the Beis Yosef that they do not, which is followed by the Badatz Edah Hachareidit. However, most shemitta related issues have no clear mesora. We have the Mishnayos, gemaras, Yerushalmi, Rambam, but we are missing almost all the intervening Rishonim and Acharonim. When you also add in the technological changes in farming, things become even more difficult.
This makes the task of a posek on hilchos sheviis very very difficult as he has to pasken basically from very early sources. This is part of what leads to the very divergent shittas on how to observe shemitta.