Simply because the board uses the original CBM chipset, and this can only address 2MB chip RAM, not more.
The limitation is in Alice. It might just be lack of support in the chip, but I've never bothered to look whether the pin out would support >2mb. It's a dram interface so row and column addresses are multiplexed, it's not as simple as counting address bits. Bearing in mind the a4000 2mb/8mb chip jumper, it's possible that if you could obtain an Alice that supported 8mb it would drop in to the design. It might need different ram though.
It's possible but unlikely that Alice would support 8mb but the motherboard and ram that commodore supplied was unsuitable. I'm not holding out any hope that anyone would find out how to make that work, even if it did exist.
The only way to fix it would be with an FPGA and then you might as well buy a mist or arcadefpga.