Where did the 10 forms of Durga known as the Mahavidyas originate from?
Why are they lesser known and not worshipped as much as the traditional 'Navadurga'?
Mahavidya are Shakti of Dash Avatara of Shiva. Most people are only aware about Dashavatara of Lord Vishnu. However Shiva Purana Shat Rudra Samhita also gives Dashavatara of Lord Shiva along with respective Shakti. And the respective Shaktis are called Mahavidyas:
Shiva Purana Shat Rudra Samhita chapter 17 "Dashavatar of Shiva with Shakti:
Nandis’vara said, “Now you listen to the ten incarnations of Shiva including that of Mahakala, as found in the Upasana Kanda.
Mahakala happens to be one of them, who provides devotion as well as the salvation to the noble people. Mahakali happens to be his Sakti, who fulfills the desires of the devotees.
The second incarnation is that of Tara having Tara as his Sakti. Both of them bestow comfort to the devotees
The third incarnation is that of Bhuvanesa and his Sakti is called as Bala. Both of them bestow comforts to the noble people.
The fourth incarnation is that of Srividyesa and his Srlvidya is known as Sodas’i. Both of them provide devotion and salvation to their devotees.
Bhairava happens to be the fifth incarnation of Siva and Bhairavi in his Sakti. They fulfill the desires of their devotees.
The sixth incarnation is that of Chinnamastaka, and his Sakti is known as Chinnamasta. Both of them fulfil the desire of his devotees.
The seventh incarnation is that of Dhumavan. Dhumavati happens to be his Sakti. Both of them fulfil the desire of their devotees.
The eighth incarnation of Shiva, is that of Bagalamukha. Bagalamukhi, happens to be his Sakti.
The ninth incarnation is that of Matanga and Matangi is the wife of Shiva, who happens to be his Sakti. She fulfills all the desires.
Kamala happens to be the tenth incarnation of Shiva, which bestows the devotion as well as the salvation. Kamala or Girija happens to be his Sakti. She protects her devotees.
These ten incarnations of Shiva, provide always the worldly pleasure and salvation to their devotees.
The one who adores the ten forms of Shiva with an umblemished and spotless mind, daily. he achieved several types of comforts.
In the Tantra literature these ten incarnations of Shiva have been described. By adoring them, all the desires are fulfilled.
0 Sage, the astonishing type of glory of these eternal Saktis, has been highlighted in the Tantra-literature, who fulfil all the desires.
In the activities like the killing of the enemies, the aforesaid Saktis are taken to be the best. They punish the wicked people and increase the Brahmateja.
O Brahman, Mahékala, and other Saktis of Shiva, have been described by me with his ten incarnations. During all the festivities relating to Siva, whosoever recites this episode, or listen to it, he becomes immensely dear to Siva.
In case a Brahmana recites the same, he achieves the Brahmaleja, a Kshatriya becomes victorious, the Vaisya achieves the wealth, while the Sudra becomes comfortable.
The followers of Shiva, while listening to this episode, become more comfortable and get more devoted to Shiva.
The detailed information about these Shaktis with their stories are avaliable on Agama Shastras especially Shakta Aagamas. As they are associated with Tantric vidhi, mostly Tantric Upasakas only worship them. So, they become lesser known to common people.