Brand loyalty, better website etc. I think the only issue I ever had with Tower's site is miscategorization. Horizon's sit is just terrible, even on it's fourth iteration.
Also like how Hobbico had Omnimodels, but eventually used it mostly for eBay sales. Keep in mind there are also a bunch of people that have no clue and don't care that Tower = Horizon, so they just buy where they have always bought.