Why do recruiters rarely give explanations to why they have rejected a certain candidate?

One or Few of the following might be the reason(s) for recruiters rarely give explanations to why they have rejected a candidate. 

• The candidate's expectations might be beyond the budget that the company has allocated for this position. 
• The recruiters might have got a better candidate who can also join at short notice and within the company budget
• Sometimes Personal preferences of the Hiring Manager which cannot be disclosed to an outsider. Ex. someone might prefer candidates from Top Colleges or from Top Companies or who have job stability. 
• The recruiter might have already made an offer to another candidate and kept this candidate as a Backup.

 

I agree with Manickavasagam Apparsamy  answer 

they are focusing on their target ( fulfill the vacant position) so they continue their search for qualified candidates  

they are only searching for the candidate meet their required needs

because sometimes they are not convinced with the reasons themselves.

They either want to avoid unpleasant reactions from the applicants, especially those who take things to a personal level, or they just don't have the luxury of time to enumerate their reasons.

Not all Candidates get rejected because they are not qualified, company needs might suddenly change due to internal factors. Sometimes, recruiters do not have time to spend on such conversations which might turn into a grudge against the firm.

In order to avoid a negative conversation where in th candidate mught get defensive.