Guy Halsall notes that both throwing axes, which supposedly characterized Frankish armies in the late Roman period, and javelins like the pila largely went out of style in the West after the collapse of the Roman Empire.

Why this occurred may be simply that the barbarian armies simply did not see the value in such weaponry post Rome. Even the Frankish throwing axe may be actually a late Roman Army artifact that by Carolingian times was no longer used.

It may be that these weapons were generally used as part of a Roman style of fighting that fell out of usage as Roman discipline and the concept of a regular, professional army disappeared. Javelins might be useful, but they were also difficult to recover post battle, and individual warriors who now had to provide their own gear instead of being provisioned by a regular army depot may not have wanted to spend their own funds on a weapon they couldn't recover.

Without a dedicated market, i.e. a Roman army ordering thousands of pilum to outfit units with standardized gear, I could see why even a useful weapon could fall out of use.

Source: Guy Halsall, Warfare and Society in the Barbarian West, 450-900