Because Allah Almighty has given preference to secret charity over open charity. Allah Almighty says: «‏‏‏‏إِن تُبْدُوا الصَّدَقَاتِ فَنِعِمَّا هِيَ ۖ وَإِن تُخْفُوهَا وَتُؤْتُوهَا الْفُقَرَاءَ فَهُوَ خَيْرٌ لَّكُمْ ۚ» ‏‏‏‏ [ سورة البقرة: 271 ] "If you give charity openly, it is good; but if you give charity secretly to the poor, it is much better for you."

Sayyiduna Abu Dharr (may Allah be pleased with him) narrates from the Noble Prophet (peace and blessings of Allah be upon him) that he said: "There are three types of people whom Allah loves, and three types of people whom Allah dislikes. As for those whom Allah loves, the first is the person who comes to a people and asks them in the name of Allah, not because of any kinship with them. Then one of those people goes behind the others and secretly gives him wealth, such that only Allah and the recipient know of his gift. The second are those who travel throughout the night until sleep becomes the most beloved thing to them, so they all dismount and sleep, but this person stands up and begins to weep before Me and recite My verses. The third is the person who is in a battle, and when the army faces the enemy, the army is defeated, but this person stands firm against the enemy until he is killed or Allah grants him victory. And the three types of people whom Allah dislikes are: an old adulterer, a poor person who is arrogant and proud, and a wealthy oppressor."