Because the power wasn't perfect.
Jack's power is communication. As such, he gets information from shards, which gives him combat telepathy. HOWEVER, it broadcasts as well as recieves, so his allies all get the same power, tilted towards protecting Jack himself from danger. However, as seen (and what, when prompted, caused Wildbow to reveal this) with Imp suddenly getting a bad feeling when about to get Jack, his enemies also get a subtle version that intimidates them from acting on chancy plans, evening the playing field. After all, if Imp DID go for Jack, she would be hosed. So the subtle master aspect to his power actually benefited her.