Why udp without flags and tcp has flags ?

UDP is without flags and TCP is with flags because TCP focuses on quality of transmission and UPD focuses on speed of the transmission.

Explanation:

The TCP(Transmission Control Protocol) first it establishes by 3-way handshake.It keeps track of every data packet.If the data packet send by the sender is not reached to the receiver it sends that data packet again after the timeout.For these functions to accomplish TCP needs flags such as SYN,ACK etc.

On the other hand UDP(User Datagram Protocol) does not focus on the quality of the transmission it focuses on the speed.It keeps sending the data packets it don't care if the data packet gets lost.Hence it doesn't needs the flags.