The primary reason Gail Kim left TNA for WWE was due to the significant discrepancy between the kind of money WWE offered her and what TNA offered her. According to multiple sources, Kim really didn't really want to leave TNA, but she felt she had no choice but to accept WWE's offer due to the massive difference in money.
Despite being the most consistent TV ratings draws for the company, it appears that no matter how successful the female performers are for TNA, they're still not going to make much money, at least in comparison to the guys. The company still saw the top babyface in the division of what has become, from a television ratings standpoint, their most successful entity, as a low-budget expenditure. TNA offered their top female babyface star well under $100,000 per year, a figure that can only be met if she appears on a target number of shows, so if she gets hurt, she's not getting all the money. TNA's offer to Kim was said to be less than one-third of what WWE was offering - a six-figure downside guaranteed contract for three years. Also, Kim can potentially earn significantly more from appearing on pay-per-views and house shows, not to mention merchandise such as action figures. Additionally, TNA's lackluster offer to Kim probably doesn't bode well for the other girls in the company when their respective contracts come up considering that the top star couldn't even get a six figure offer.
Kim's friends had to talk her into leaving for WWE due to her strong sense of loyalty and happiness with TNA. While Kim loved TNA as well as her friends in the company, she couldn't justify not taking the significantly better offer from WWE. One friend of Kim said she really had no choice but to go for the financial security that only WWE can provide. One TNA source disappointed in her decision said she chose to be unhappy for more money rather than stay happy with the smaller company that made her a star.
There was said to be a huge void backstage at this week's iMPACT! tapings as a number of her friends couldn't believe she was gone, especially without saying goodbye before she left.
One friend of Kim's with knowledge of TNA's apparent lowball offer deemed it to be "insulting" and "possibly sexist.
Source: 24wrestling