To avoid l-3 loop we have TTL in IP header. why there is no TTL field in Ethernet header to avoid l-2 loop .we have separate protocol.why?

There are two types of loop data plane and control plane.

TTL value is used to prevent data plane loops in network where as STP is used to prevent control plane loops in network.

Routing protocols have their inbuilt mechanism for preventing control plane loops like EIGRP has DUAL , OSPF has sequence number.