To TheApostleAK:
The basic reason is that it fouls up the Society's notion of Bible chronology. Note the following: According to Jer. 46:2, Nebuchadnezzar defeated Egypt at the battle of Carchemish in the 4th year of Jehoiakim; according to Jer. 25:1 Jehoiakim's 4th year was Nebuchadnezzar's 1st year; according to the rest of Jer. 25, the Babylonians had not yet attacked Judea when Jeremiah wrote this chapter; and according to an undisputed Babylonian cuneiform tablet Nebuchadnezzar defeated Egypt at Carchemish in his accession year (note that this is according to Babylonian dating, which dates the reign of kings from Nisan to Nisan, in early spring, and uses the accession year system; I'll call this NN-AY dating) and took possession of all of Palestine. Thus, the earliest date at which the Babylonians came against Judea is in the 4th year of Jehoiakim (apparently using Tishri-Tishri dating and the non-accession year system; I'll call this TT-NAY dating). But if Daniel 1:1 refers to the 3rd year of Jehoiakim (again using Tishri-Tishri and non-accession year dating), then there is a problem with Bible chronology since other passages indicate that the Babylonians first attacked in Jehoiakim's 4th year.
If we lay aside this problem for the moment and consider the dates at which the secular and Watchtower chronologies put the main events, the Society's problem becomes clear. According to secular chronology, the battle of Carchemish occurred in spring to early summer of 605 B.C.E., between April 12 and August 15 at the outside, and Nebuchadnezzar ascended the throne on Sept. 7, 605 B.C.E., in the month Elul, which is just before Tishri. Therefore the earliest date that any Jews could have been taken captive to Babylon was 605 B.C.E., and since the Watchtower Society claims that the Jews were captive in Babylon for 70 years, from 607 to 537, this obviously doesn't work. What about using the WTS date for the 4th year of Jehoiakim, i.e., 625 B.C.E.? Well that doesn't work either, since Dan. 1 clearly indicates that captives were taken to Babylon in the 3rd (and not later than the 4th) year of Jehoiakim. From 625 to 537 is 88 years, and this is inconsistent with WTS claims.
Furthermore, Dan. 2 indicates that the training of Daniel and his companions in the language and customs of Babylon was completed in Nebuchadnezzar's 2nd year, which by secular dating is about 603 B.C.E. (using NN-AY dating) and is about 624 B.C.E. by WTS dating (using TT-NAY dating). Again neither of these dates fits in with WTS chronology.
In view of the above discrepancies, the Watchtower Society had to assign odd interpretations to Dan. 1:1; 2:1 in order to retain its tradition.
According to information in Carl Jonsson's book The Gentile Times Reconsidered, it's easy to reconcile apparent discrepancies in the Biblical dating in Daniel. Since Daniel became a high political official in Babylon, it would have been natural for him to use the Babylonian dating method to date various events. Thus, the Biblical "4th year of Jehoiakim" would be approximated by the 3rd year in Babylonian style dating, keeping in mind that the systems were also different by six months. Thus, Nebuchadnezzar's ascension to the throne on September 7th of 605 would be in his ascension year, which in Babylonian dating ran from Nisan 1 (April 12) 605 to Nisan 1 (April 2) 604 B.C.E., whereas in Jewish dating it would have been in his first year, running from Tishri 1 (Sept. 19) 606 to Tishri 1 (Oct. 7) 605 B.C.E.
Given the above, after the battle of Carchemish the Babylonians could easily have come against Judea in spring to summer of 605 B.C.E, in "the 3rd year of Jehoiakim" (NN-AY dating) and in Nebuchadnezzar's accession year, taken Daniel and others captive, so that by the end of Nebuchadnezzar's 2nd year (NN-AY dating) they would have been captive for nearly three full years, running at the outside between April 12, 605 and April 10, 602 B.C.E. Therefore the statement in Dan. 1:5, that Daniel and his companions were to be trained for "three years", presents no problem at all for secular dating, contrary to claims of Watchtower apologists who prefer to ignore the evidence presented above.
AlanF