Maybe I'm too cynical, but I'd bet it's a financial thing. Tak probably just said "Use whatever you want in the studio, but use our stuff in print/videos and we'll give you money" I have a hard time believing Martin, Gibson, whoever else couldn't make a guitar every bit as good a Takamine for electric playing.
The electric EQ on Gibsons and Martins have been excellent for many years now and there's nothing wrong with them. I think maybe it's a copycat thing. Why do so many country players use Telecasters? There are so many guitars that can get that sound but it's just a cliche, and butterscotch on top of that. To me it would just look weird these days to see a country electric player with a Jackson, PRS, Schechter, strat or Les Paul or playing acoustic with Martin, Gibson, or Ovation. While there are country stars with lots of other stuff, the Taks and Teles seem so prevalent it's almost as if Takamine and Fender gave all the country artists endorsement deals they couldn't deny. The only other maker I saw get a lot of inroads in country disproportionately compared to the guitar market were Peavey guitars in the 1980s.
When I watch American Idol all of a sudden one year every acoustic player had a Taylor. It became obvious at some point that there was an agreement made between the TV franchise and Taylor Guitars.
Ibanez is huge in metal now and in the past and I don't think it's so much getting paid by Ibanez but the Wizard necks and Edge tremolos. You can get a good shredder workhorse in a Jackson, Charvel, Schechter, ESP, or Dean but at every price point, Ibanez seems to get the metal vote more than any other, especially for instrumental metal. There was a time that Shrapnel Records seemed to be an Ibanez advertising campaign.
I love Ibanez but if I had a Jackson Dinky USA Series, Charvel Govan, ESP MII, or Dean Mustaine I wouldn't complain and if somebody can't play and sound good on those, it's on the player not the guitar. Ibanez goes the extra step in the tiny details and thus the popularity of playability of a Prestige RG.