Why does the MAX3420E not support ISOCHRONOUS transfers?

ISOCHRONOUS transfers require fast interfaces and large buffers, neither of which is consistent with the MAX3420E design goals (a low-cost part with an SPI interface that can run at any speed). Most applications that seem to require ISOCHRONOUS bandwidth can actually be handled with BULK or INTERRUPT transfers. This is because most of the ISOCHRONOUS bandwidth available to a USB device in a system is also typically available for BULK/INT transfers.