His original proposal video[0] explains it well: it's not supposed to be powerful; he is aware of the Raspberry Pi but doesn't see it fitting his vision. It's supposed to be something like the C64 and other computers of the era where you could understand how all the various chips interacted and worked.
Things like DDR memory abstract all that stuff away. Computers today don't "put an address on the bus and read the data" like parallel SRAM/DRAM; they send a command packet to the memory chips and wait for the data packet to come back. Audio interfaces, especially over PCIe, aren't as simple anymore either. You can't write direct to the screen anymore like you could with VGA cards on old IBM PCs. All these abstractions make computers so complex that one person can't possibly understand it all at once.