Why do Muslims have two religions like Ahl al-Sunnah and Ahl al-Hadith?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Islam is only one religion. Its sources are the Qur'an and the Sunnah of Allah's Messenger (Sallallahu Alaihi wa Sallam). It is required to understand them as done by the companions of Allah's Messenger, who witnessed the revelation of the Qur'an in their language, comprehended reasons of revelation and lived up the teachings of the Qur'an with the Prophet (Sallallahu Alaihi wa Sallam) and as Allah and His Messenger ordered.
Here, we state that those Muslims who adhere to the above-explained way of life are called Ahl al-Sunnah (the followers of the Prophet's (Sallallahu Alaihi wa Sallam) practice) or Ahl al-Hadith. In other words, "Ahl al-Sunnah or Ahl al-Hadith" means each and every person who follows the right. The religion of Islam only.
When Muslims were faithful to Islam, they were known only by this name, i.e. Muslims. One exception is that Muslims, for some actions, were called al-Muhajireen, al-Ansaar and al-Badrieen and so on. But, some Muslims deviated from the Islamic way of life and became sects and parties as happened to Jews and Christians as stated in the Prophetic Hadith: "Jews were broken up into seventy-one or seventy-two sects, and Christians were so, and my Ummah (Muslims) will be divided into seventy-three sects" [[ Ahmad , Abu Dawood and al-Tirmizi ].
Then, followers of the Qur'an and the Sunnah are called Ahl al-Sunnah and Ahl al-Hadith to be distinguished from the followers of Bid'ah (innovation) such as al-Khawarej, al-Murjeeh, al-Mu'tazilah and the like.
Allah knows best.