Low frequency sound requires a larger and more powerful source, plus it is more difficult to localize. ELTs have severe size and power limits, so a high frequency noise to pinpoint the source makes sense.
The ELT design requirements may now get another look, as very long range trans oceanic flights have become much more frequent and the ELT is becoming more critical. However, given the unhappy experience recently with a runaway ELT damaging a 787 at Heathrow, people are unlikely to embrace bigger, more powerful ELTs. The insurance industry will work with the aircraft makers to come up with a better system, something that leaves a reliable breadcrumb trail for every flight.