It's all about mergers & acquisitions, Badger. TNN was purchased by CBS, which sorely needed an outlet on the cable dial to play catch-up with ABC & NBC.
CBS's plan was to turn TNN into a top-10 cable network to compete with TBS, TNT, and USA. They began to move away from the niche market of country music, fishing/hunting/outdoors shows, and motorsports. They figured that country music fans would be just tickled pink with CMT, and that the oudoors & racing fans could get their fill on ESPN & ESPN2 & Fox Sports.
CBS was later acquired by Viacom. Viacom also owns Nickelodeon, MTV, VH1, TV Land, and several other related networks. Before long, TNN changed from The Nashville Network to The National Network ... continuing the process started by CBS. The thing was, the audience demographics weren't what CBS originally planned. They were skewing younger, and predominently male.
Well, before long some corporate muckity muck asked 'if Lifetime is "television for women", why not have "television for men"?' And, Voila! We now have Spike TV, which is very heavily targeted toward a younger adult male audience.