Why are license plates for bicycles no longer used?
In the Netherlands at least:

The license plates for bikes were tied to the taxation of bikes. You needed to get a new plate every year.
The taxation lasted until WWII. The Germans abandoned the tax.

After the war the tax wasn't introduced again so the plates were no longer needed.

It was speculated that after the war the taxing of cars more then made up for the not taxing of bicycles.