Because of this saying of Allah Almighty: "So let them not approach al-Masjid al-Haram after this year of theirs." [Surah al-Tawbah: 28]
And this is also the view of Sa'id bin al-Musayyib (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q4330
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) sent a cavalry detachment towards Najd, and they brought back a man from Banu Hanifah called Thumamah bin Uthal. They tied him to one of the pillars of the mosque.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4330
Anas bin Malik (may Allah be pleased with him) said: A man came in on a camel; he made the camel kneel in the mosque, tied its leg, and said: Which of you is Muhammad (peace and blessings be upon him)? The Prophet (peace and blessings be upon him) was sitting reclining among the Companions (may Allah be pleased with them). The Companions (may Allah be pleased with them) said: This fair-complexioned man who is reclining. He said: O son of Abd al-Muttalib! He (peace and blessings be upon him) said: "Yes, I am listening to you." He said: I have come to ask you a question.
In another narration, the man's name is Dimam bin Tha'labah. Al-Layth said: He made his camel kneel near the door of the mosque, tied its leg, and then entered the mosque.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4331
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Jews came to the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was sitting with his Companions in the mosque. They said: O Abu al-Qasim, what is the ruling concerning a man and a woman who have committed zina (fornication)? (and the rest of the hadith).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4332
Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him) said: I came to Madinah concerning the ransom of the captives of Badr, and at that time I was a polytheist. I entered the mosque while he (peace and blessings be upon him) was leading the Maghrib prayer, and in it he (peace and blessings be upon him) recited Surah al-Tur, and it was as if my heart were being split apart by the recitation of the Qur'an.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4333
Sayyiduna Uthman bin Abi al-As (may Allah be pleased with him) said that the delegation of Thaqif came to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) lodged them in the mosque so that it would soften their hearts more. They set conditions on the Prophet (peace and blessings be upon him): that they would not be mustered (for military campaigns), that a tenth would not be taken from them, that they would not be made to bow (in prayer), and that no one from outside their own people would be appointed as governor over them. He (peace and blessings be upon him) said: "You will not be mustered, a tenth will not be taken from you, you will not be made to bow, and no one from outside your people will be appointed over you; but there is no good in a religion in which there is no bowing (ruku)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4334
Sayyiduna Uthman bin Abi al-As (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) lodged Banu Thaqif in a tent in the mosque so that it would soften their hearts more. Then he mentioned the hadith about the conditions they set when they accepted Islam.
(b) Ash'ath narrated it from al-Hasan (may Allah have mercy on him) in mursal form with part of its meaning, and added: It was said: O Messenger of Allah, you have lodged them in the mosque while they are polytheists? He (peace and blessings be upon him) said: "The son of Adam becomes impure; the earth does not become impure."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4335