Here's why:
* Tom is not particularly observant or perceptive. He's more concerned with maintaining his own image and power. He doesn't spend much time trying to figure out Gatsby's business dealings.
* Tom relies on rumors and speculation. Tom's primary source of information about Gatsby is through gossip and whispers. He never confronts Gatsby directly about his business.
* Tom's primary focus is on Gatsby's relationship with Daisy. Tom's jealousy and possessiveness towards Daisy blind him to Gatsby's true nature. He's more interested in exposing Gatsby's affair with Daisy than his illegal activities.
* Gatsby's wealth is not necessarily tied to bootlegging. While Gatsby's wealth is suspicious, it's never explicitly confirmed in the novel that he makes his money through bootlegging. He's portrayed as someone who could be involved in any number of shady business dealings.
Instead of knowing Gatsby is a bootlegger, Tom believes Gatsby is a "phony" and a "liar" due to his extravagant lifestyle and mysterious past. Tom sees Gatsby as a threat to his own social standing and his relationship with Daisy.
It's important to note that while Tom doesn't explicitly know Gatsby is a bootlegger, the reader is aware of this through various hints and clues. The novel's atmosphere and setting, with its constant portrayal of wealth and excess, strongly suggest that illegal activities, like bootlegging, are a significant part of the world Gatsby inhabits.