As
Muhammad The Conqueror is called conqueror because he conquered Constantinople. (Istanbul)

why Sulayman the Magnificent called the magnificent? is it becoz he give Laws or Qanuns.
what is the basic thing that distinguish him as The Magnificent.

A.w.A

You rightly pointed out that SULAIMAN was a lawmaker bacause of the fact that he reconstructed the OTTOMAN legal system completly and for that matter he comes to be known in the EAST as a lawgiver.Thus his title of lawgiver has rightly been justified.As far as he being a "magnificent" is concerned,I think it is because he was the longest reigning OTTOMAN CALIPH from 1520 untill his death in 1566.He led his armies personally to conquer the CHRISTIAN strongholds of BULGRADE,RHODES and HUNGRY untill his conquests were checked at SIEGE of VIENA.Another point of concern is that under the prestigious rule of SULAIMAN,the OTTOMAN fleet held sway over the seas from MEDITERRANEAN to RED SEA andPERSIAN GULF.MIDDLE EAST was made subordinate during his rule and same happened with the considerable portion of NORTH AFRICA as far WEST as ALGERIA.
The above mentioned achievements will prove helpful to explain this very fact that why SULAIMAN is known "SULAIMAN THE MAGNIFICENT" in the WEST.

to my mind, answer is this......
Sultan Suleiman undertook a reform and simplification of the law, finally producing the famous code known as the “Ottoman Law” which was to be the law of the land in the empire for the next 300 years. He improved conditions for the Christians and Jews within his empire and made criminal punishments less cruel and decreased the number of capital offenses.
It is a mark of the greatness of the reign of Suleiman the Magnificent that he was widely admired by friends and enemies alike. To his own people and his allies he was known as a wise and just ruler while his enemies had to admire his talent, wealth and power. he was the 2nd person who expanded the size of his territory in history on large scale after alexender the great.....
so conclusion is this, he was law giver and his made law was suitable for the non musilms such as jews and christians and for other non muslims, secondly he was very tolerant towards his subject with regard to religion t 3rdly he was very wise and talented 4thly he expanded territory to great extent therefor people of europe realized that he is magnificent (means the great). however "the great" is title, the world gave to alexender due to establishing of vast empire and magnficent to suleyman who doubled the ottomon size