Many explorers "cherry-pick" - they don't bother scanning anything except the highest-value objects: neutron stars, black holes, ELWs. No scans, no Tags.
I was exploring over in Qeajo sector a few months ago; it's off the regular tourist routes, not on a direct line to anywhere. Saw no other CMDR tags for a thousand LYs; saw no ELWs either. Finally found my first ELW in that sector... and it was already Tagged.
awwww damn!!! thats some crap..