This new design saw the motherboard flipped to the other side of the case, meaning the graphics-card cooler expelled hot air upwards, rather than down into the adjacent expansion cards.
The processor was moved nearer the front intake fan and the north bridge placed behind it, along with the DIMM sockets. Incompatible with ATX parts, manufacturers were reluctant to embrace BTX until the consumer demand was there, but it failed to materialise.
In the meantime, heatsinks grew more efficient, as did processors, diminishing the need for better airflow.
Worse still, the rigid guidelines on motherboard layout meant parts with the memory controller on the processor, such as the hugely popular Athlon 64 and more recently, the Core i7, wouldn't fit on a BTX board at all. BTX parts can still be bought, but its shot at the mainstream is long gone.