Why isn't the Jubilee year being kept anymore? Why can't we keep the commandments and regulators regarding this year being uphold these days?
Rambam Hilchos shmitta Halacha 8 (below) says that Yovel is not in effect unless all twelve tribes are on their land. That is why we stopped the yovel once the tribes of Reuven, Gad, and the part of Menashe that lived on the eastern side of the Jorden were sent into exile.
There are those that say that the Shmitta is rabbinic only as a result of this. That is why we have the leniencies that we do nowadays.
There is a difference of opinion in the Talmud as to whether the jubilee year was included in or excluded from the forty-nine years of the seven cycles. The majority of rabbis hold that the jubilee year was an intercalation, and followed the seventh Sabbatical year, making two fallow years in succession.After both had passed, the next cycle began. They adduce this theory from the plain words of the Law to "hallow the fiftieth year," and also from the assurance of God's promise of a yield in the sixth year sufficient for maintenance during the following three years, "until the ninth year, until her fruits come in" (Lev. xxv. 22), which, they say, refers to the jubilee year. Judah ha-Nasi, however, contends that the jubilee year was identical with the seventh Sabbatical year (R. H. 9a; Giṭ. 36a; comp. Rashi ad loc.). The opinion of the Geonim and of later authorities generally prevails, that the jubilee, when in force during the period of the First Temple, was intercalated, but that in the time of the Second Temple, when the jubilee was observed only "nominally," it coincided with the seventh Sabbatical year. In post-exilic bṭimes the jubilee was entirely ignored, though the strict observance of the shemiṭṭah was steadily insisted upon. This, however, is only according to a rabbinical enactment (Tos. to Giṭ. 36a, s.v. "Bizeman"), as by the Mosaic law, according to R. Judah, shemiṭṭah is dependent on the jubilee and ceases to exist when there is no jubilee (Giṭ. l.c. and Rashi ad loc.).
This is part of why Hillel could make the takkana about pruzbul while the second temple was in existence. Rambam states (Halacha 7) that when the Yovel was in effect, the 49th year of a cycle was the shemittah, year 50 was the Yovel, and year 51 was the start of the next cycle. Halacha 3 says that they did this during the second temple era even though they did not observe the Yovel. Halacha 5 says that when the temple is not in existence, then only the seven year shmittah cycles are counted and the youvel year is not counted. That is why we divide the year number by 7 and get the number of the shmitta cycle year (5775%7=0 - this is a shmittah year).
From the time the tribes of Reuven and Gad and half the tribe of Menasheh were exiled,26 [the observance] of the Jubilee year ceased, as [implied by Leviticus 25:10]: "You shall proclaim freedom throughout the land to all of its inhabitants." [One can infer that this commandment applies only] when all of its inhabitants are dwelling within it. [Moreover,] they may not be intermingled, one tribe with another, but rather each tribe is dwelling in its appropriate place.27
When the Jubilee is observed in Eretz [Yisrael], it should also be observed in the Diaspora,28 as [implied by the phrase used in the above verse:] "It is the Jubilee," [i.e.,] in every place. [This applies] whether the Temple is standing or whether the Temple is not standing.29