It is because of the great similarity between what happened to Prophet Musa (AS) and Prophet Muhammad (SAWA) and what happened hy and to the community to whom Prophet Musa was sent, and what will happen by and to Muslims who are supposed to follow the teachings of Prophet Muhammad (SAWA).
Allah (SWT) did mention in Quran (Verily, We have sent to you (O mankind) a Messenger (Muhammad) who is a witness on you, as We have sent to Fir'aun a Mesenger (Musa). Sura 73, Verse 15.
We don't read in the whole Quran any verse like this verse of similarity. Allah (SWT) never mention similarity between Prophet Muhammad (SAWA) and any other prophet like it is between him and Prophet Musa (AS).
The verses mentioning the sufferings of Musa from the disobedience from his own community (Bani Israel ) are warning the Muslims from similar attitude of disobedience of the orders of Prophet Muhammad (SAWA). Allah (SWT) said in Quran ( O you who believe, Be not like those who annoyed Musa, then Allah saved him from what they claimed ( against him) Sura 33, verse 69).
In Sura Al-Jum'ah which is recommended to be recited on Friday Prayers and many other prayers on Thursday night and Friday morning, we read how Allah (SWT) is warning the Muslims from the fate of those who were claiming believing in Prophet Musa (AS) and were been asked to follow Torah, but they failed to obey. Allah mentioned them as an example for Muslims and said they are like a donkey carrying books which means that Muslims who will not obey the Prophet and Quran will be like these who denied the signs of Allah.
Allah (SWT) did mention in Quran that the successors of Musa are twelve ( Indeed, Allah Has taken the covenant from Bani Israel (Musa's community) and We appointed twelve (divine) leaders . Sura 5, verse 12. This verse and above verse (Al-Muzzammil: 15) confirming the similarity between Prophet Musa and Prophet Muhammad (SAWA) should be enough evidence to prove that the successors of Prophet Muhammad (SAWA) are twelve. This Quranic evidence confirms many authentic prophetic statements mentioning the Twelve successors of the Prophet Muhammad (SAWA) which are mentioned in Shia and Sunni books of Hadeeth including Saheeh Bukhari : 6682, Saheeh Muslim : 3393, Musnad Ibn Hanbal :3593, Tirmithi; 2149 and many other Sunni books.
Prominent Sunni scholar from Al-Azhar in Egypt ( Shaikh Mahmoud Abu Rayyan) who did thorough research comparing between different Muslim sects said that if Muslims understand Quranic verses about Musa and Harun,they will understand that none than Ali (AS) is the real successor after the Prophet Muhammad (SAWA).
This subject has many details which can lead Muslim Ummah to unity under the orders of Allah (SWT) and the Prophet (SAWA).
Wassalam.
