The reason s basically what they've been saying for the past few years now: you can't put a 10-year-old boy as a figure on shelves at retail because it's not going to 'sell'. Now keep in mind that they've just released a young Ofelia from Pan's Labyrinth (amazing line btw) on shelves and also another young child from The Devil's Backbone as part of their Guillermo del Toro line. But JC with his dirt bike as a large accessory isn't going to sell?
Their biggest blunder, however, was advertising him on the fucking back of the packaging for the recent Kenner Terminator releases, giving people a false sense that he was going to be coming to retail. Newt from Aliens was a SDCC exclusive, sold good numbers and they had to put her in a two-pack for retail afterwards with their new cocaine jaw Ripley head sculpt. So he might be released in a two-pack with Arnie later on but that means we won't get the bike.
It also looks like they scrubbed the GnR letters off the back of the bike since he was last shown. Probably couldn't get permission from whomever (Guns n Roses I presume?).