Various reasons have been suggested. For a start, Bartlett (1998) says the rebels were unable to take Dublin at the outset. This deprived the rebellion of a "focus" and prevented also the formation of some sort of credible, representative assembly.
The "staggered outbreak" of the rebellion was also said to have played into government hands. One loyalist said he was eternally "thankful" that the "insurgents" had acted "so little in unison" meaning that they could be beaten off separately one by one.
The counties round Dublin had risen first and been quashed. A few days later Wexford rose but that was sealed off. Then Antrim and Down were subdued. It was only in August/September that Mayo, Sligo and areas in Longford and Westmeath responded to General Humbert's "invasion." But by then, Bartlett (1998) says, it was far too late.
The third explanation relates to the French themselves. Their failure to intervene in large numbers in the summer of 1798 is seen as decisive. A substantial French force would have provided "discipline, leadership, weaponry, recognition" and -above all- a badly needed, overarching strategy.
Then finally it is suggested that Dublin Castle prevailed when it came to the "intelligence wars." Agents were said to have penetrated the ranks of the United Irishmen. Dublin Castle often knew what was going to happen in advance and was able to take appropriate preventative measures. James Hope, the widely revered Antrim rebel, would later reflect that "we were all beset by spies and informers."