Derecognition of more than 1600 Vocational Training Providers (VTPs) under the Skill Development Initiative (SDI) scheme by the Government of India was primarily due to fraud complaints. The government took this step to ensure the quality and credibility of vocational training programs provided by these institutions.

Reasons for derecognition:

Fraud complaints:
- The main reason behind the derecognition of VTPs was the presence of fraud complaints against them. These complaints indicated that these institutions were involved in fraudulent activities such as issuing fake certificates, misappropriation of funds, and providing substandard training.
- By derecognizing these VTPs, the government aimed to maintain the integrity of the vocational training system and protect the interests of the students and job seekers.

Lack of infrastructure:
- While lack of infrastructure could be a contributing factor in some cases, it was not the main reason for the derecognition of VTPs under the SDI scheme. The government's primary concern was the fraudulent activities conducted by these institutions.

Inadequate curriculum:
- Inadequate curriculum could be a factor that affects the quality of vocational training programs, but it was not the primary reason for derecognition. The government's focus was on addressing fraud complaints and ensuring the credibility of the vocational training providers.

Absence of trainers:
- Similarly, the absence of trainers might affect the quality of training, but it was not the main reason for the derecognition of VTPs. The government's priority was to tackle fraud complaints and maintain the reputation of the vocational training system.

Conclusion:
The main reason for the derecognition of more than 1600 Vocational Training Providers (VTPs) under the Skill Development Initiative (SDI) scheme by the Government of India was fraud complaints. The government took this step to protect the interests of students and job seekers, and to maintain the credibility and integrity of the vocational training system. While other factors such as lack of infrastructure, inadequate curriculum, and absence of trainers may have also contributed to the decision, they were not the primary reasons for the derecognition.