As /u/LegalAction said in his comment here, you'll have to ask him that--none of our texts actually tells us anything, so all the reasons scholars suggest are nothing but conjecture. Various conjectures have been raised, some of which I mention in my comment on the same thread, but they're still all conjecture.

I'd like to dispell this idea, though, that Caesar's troops and staff had some sort of "fanatical loyalty" towards him. Caesar's subordinates, like Caesar himself and the very people he was fighting, were very complicated and each one was unique. Many were there for personal gain, many were there for political or moral reasons. There's this idea that's become common since Mommsen, one that's been largely discarded among classicists (even Cary and Scullard didn't take it seriously in their textbook) that the Roman legions were these sort of mindless shades completely loyal and dependent on their generals and supporting them and their programs wholeheartedly. That's hogwash. The legions, like the citizen body at large, had their own agendas and were perfectly willing to enter into dispute with their own leaders in order to obtain them. Sulla's troops sided with Catiline, who, despite having originally been a Sullan, proposed some decidedly un-Sullan legislation and gathered supporters under the guise of a populist. The constant defections back-and-forth between Triumvirs, Republicans, and Pompeians following Caesar's death should show this quite readily. Labienus was not the only Caesarian to defect. He was perhaps the most important, at least for his military abilities in the upcoming war, but he was not even the most prominent. That distinction would have to go to Quintus Cicero, Marcus Cicero's younger brother, who was a diehard Caesarian and prized both for his undeniable military skill and his connections to his brother--Quintus went over to Pompey, even though from what we can tell he disagreed quite strongly with his politics and Cato's. Not even the legions were exempt--the centurion Titus Puleio, a veteran mentioned for his bravery in the de Bello Gallico (it's still usually thought, as far as I'm aware, that he's the same Titus Pullo Caesar mentions there, and that there's some textual error--various manuscripts give different versions of his name), led two legions over to Pompey from Illyria. Like with Labienus, Caesar is notably silent about the causes of this defection--he just mentions that Puleio was at Pharsalus defending Pompey's camp, and mentions in passing that he had betrayed Gaius Antonius (Antony's brother, who had been sent to command the two legions that were led over to Pompey)--so we don't really know why they defected, but the fact remains that they did. Later some of Caesar's Pompeian legions would flip-flop back and forth in Spain, although perhaps this is to be expected--still, oddly, by the time of Caesar's campaign against the Pompey brothers it can't really be said that anyone expected the Pompeian to win, since Pharsalus and Thapsus had already happened, so the behavior of these legions in siding with what was, in all likelihood, bound to be the losing side is a bit bizarre if we suppose the legions only sided with the most powerful. The Ninth legion led a mutiny against Caesar during the civil war, although they were humiliated and disgraced, and demanded to remain in service to make up for it. And this is only mentioning his military staff and troops--his political supporters in the city were completely divided, and even those who stayed behind in support of him often ended up opposing him when they thought he was doing something they didn't like (for example, during Antony's proconsular command in Italy). So it's not really fair to say that Caesar's troops or supporters were mindlessly loyal to him, any more than it's fair to say that any general's troops were--these people very much had minds of their own and were willing and able to make their desires felt

I'm a layperson, and someone with more knowledge than I may come and give their answer - but I hope I can be of help.

I'm working out of the same book that you mentioned in your post, Goldsworthy's Caesar - primarily pages 380-383.

Goldsworthy mentions that scholars have put forth conjecture that Labienus had held the praetorship before coming to Gaul, if this is correct, then he would have been at least 50 by the time of the Civil War and eligible for the position of consul for many years. In effect, this would mean that Labienus put his entire career on hold for the time that he was fighting in Gaul. Goldsworthy is quick to mention that there is absolutely no evidence that this was the case.

Another possible reason put forth by Goldsworthy is that due to Labienus' successes in Gaul - he was easily Caesar's best subordinate - he would have certainly won a triumph if he had held the governorship himself. Caesar expressed his favor for Labienus in his account of the campaigns, "Compared to the other legates, Labienus was granted a more prominent place in the Commentaries"

Goldsworthy is quick to point out that Caesar was much more forthcoming with money than he was with glory for his subordinates, and that Labienus had become very wealthy, very quickly. It is possible that Caesar intended to run for the consulship in 48 with Labienus as his co-consul, but this was never to be.

A few other reasons posited by Goldsworthy include the fact that Labienus hailed from Picenum, a region firmly "dominated by Pompey's family" and may have reverted to earlier loyalties. Possible service with Pompey is also conjectured, but again, there is no evidence to support this.

One final theory put forward by Goldsworthy is that Labienus was simply dissatisfied with what had been given to him for his years of service. As Goldsworthy puts it "Labienus had given a large chunk of his best years to Caesar and seems to have felt that this had not been sufficiently recognized." Perhaps he felt as if it had been his efforts, and not Caesar's, that allowed for Caesar's unprecedented successes in Gaul, and that he wasn't properly recognized for this. Goldsworthy mentions that some sources paint Labienus to be particularly abrasive and "was by no means a likable man." Perhaps he reasoned that the Civil War would be lost by Caesar, especially without the benefit of Labienus' admittedly spectacular talent for command.

I hope this is a satisfactory answer to your question - Labienus was a very successful commander. The most successful of Caesar's legates in Gaul. If this answer isn't any good, I'd love to know what I can do to improve.