Earth Fault

Er, because it doesn't need to be any higher, perhaps?

If you've got an earth fault you want the quickest disconnection. Overload can be tolerated for short periods, hence the higher settings.

Two Reasons:

i) First and foremost is human safety! Even a small earth fault current can create dangerous touch voltages to be developed on exposed and/or extraneous conductive parts, thereby endangering a human life. So, it is imperative that the earth fault protective setting is kept as low as possible. The 10 to 40% setting normally given is for giving due allowance for the system unbalance current - which can occur due to normal load operating conditions too (due to unequal distribution and/or utilisation of single phase loads spread across the three phases) - particularly, if the earth fault relay is wired in the residual path of the phase CTs.

ii) Second is economical! Any earthfault current is drained into the earth without doing any useful work. You wouldn't want to continue draining costly electricity! Would you?

Just to add to above, a Ph-Ph fault current flows through conductors intended for actually carrying currents, while a Ph-E fault may also flow through those 'conductors !!!' which are not intended for actually carrying currents & thus Ph-E fault needs to be isolated at lower current values than other faults such as Ph-Ph fault.