I'm speculating but, it's likely due to board size constraints and pricing. AM57xx SoC have 2 different DDR3 memory controllers called the EMIFs. Each one has

I'm speculating but, it's likely due to board size constraints and pricing. AM57xx SoC have 2 different DDR3 memory controllers called the EMIFs. Each one has a 32 bit data width and can support up to 2GB of memory attached. A single x16 (16 bit wide data path) DDR3 chip in 512MB (4Gb) size is reasonably cheap to buy today but doubling that size to 1GB (8Gb) makes the price go up like 4x since there's just not much volume in that size sold. In order to fit onto the BeagleBone board size, only 2 DDR3 chips could fit, so only 1 EMIF is used, and to keep costs reasonable they used 512MB (4Gb) sized chips leading to 1GB of total DDR3. Related, the C66 DSPs and M4 cores inside of AM57xx are only able to access the EMIF interfaces over the L3_MAIN core interconnect within the SoC. L3_MAIN is a 32 bit address bus interconnect, so at most can address only 4GB of memory address space. TI's memory maps for the DSPs and M4 cores both start the EMIF addressing at 0x8000_0000 so at most the DSPs and M4 cores can only access up to 2GB of DDR3. Addresses below 0x8000_0000 are mostly memory mapped peripherals, like the GPIOs and PCIe interfaces and such. The Cortex-A15 cores are able to use an extended addressing mode and can access the EMIFs through a different interface, they don't have to use L3_MAIN (but can, but there's errata), so if you connect more than 2GB of DDR3 to AM57xx then the big Cortex-A15s can access it but no one else can, which makes it less useful/valuable for a lot of designs. If you want a dev board with more DDR3 and AM57xx, the BeagleBoard-x15 is an option although it appears to be hard to buy just the x15 from most distributors right now as they're out of stock. TI still sells the AM572x dev kit with the LCD screen directly (https://www.ti.com/store/ti/en/p/product/?p=TMDSEVM572X https://www.ti.com/store/ti/en/p/product/?p=TMDSEVM572X), albeit for a bit more than just the x15 used to cost and quite a lot more than the new AI costs.