Tom loves Daisy and not Myrtle because Daisy belongs to the same social class as him. While she might not come from as much money, as Nick makes sure the audience knows that Tom is filthy wealthy (he always mentions the polo ponies), she runs in the same social circle. To the wealthy, at least according to Nick, where one comes from matters more than the money one has. For example, Gatsby, regardless of how much money he had, could never have Daisy, as evidenced by her rejection of the parties held at Gatsby's home.
To Tom, Myrtle was just his living sex toy he liked flaunting around town. When taking Nick to meet Myrtle, Tom shows no shame and tells Nick (emphasis mine), "I want you to meet my girl." While this might just seem a throw-away statement, Tom's actions later in the night, when he "broke her nose with his open hand," show a complete disregard for Myrtle as a person. In addition, it seems as if after this event that Tom does not really see Myrtle, his "girl," again.
Meanwhile Daisy, whom Tom shares similar moral standards with considering they come from the same social class, does not really seem to care what happens to Gatsby, a man she seems in complete love with. Like Tom, Daisy completely discarded Gatsby after she ran Myrtle over.
I bring up Daisy in response to your question in order to emphasize why Tom loves her, but not Myrtle. They share the same moral values: carelessness resulting from being American aristocrats. Nick summarizes the values of Tom and Daisy this way:
"They were careless people, Tom and Daisy—they smashed up things and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mass they had made..."
After Tom smashes Myrtle's nose and literally lets others clean up her mess, he retreats back into his marriage with Daisy, who does the same when George Wilson kills Gatsby.