Not sure. SRX is almost ten-year-old, albeit still relevant technology.
Someone mentioned how ARX boards generate a lot of heat (which is likely why there are only two slots), and maybe SRX boards would just add to that? A few of the XV devices were transitional; that is, they had both SRX and JV-80 slots, but if the heat's too great for both ARX and SRX, Roland had to make a decision.
And who knows? ARX boards are synths in their own right. Perhaps Roland could create a huge 256MB ROM board on a chip. Or a 4-voice VA. I'd greatly prefer that over SRX any day, especially considering how quite a few SRX patches are already built in.
But the biggest reason I see is this: If you already have a synth that takes SRX boards, the stock sounds won't automatically appear in the G, so you'll need to keep it to recreate old songs anyway. If you don't have a synth that takes SRX boards, you won't know what you're missing.