Why in most of the companies, position of "Chief Financial Officer" and "Company Secretary" is held by the same person ?

Cause this person is well informed about all transactions in and out and the objectives of the organization. 

It happens in some companies 

Because the nature of the work of the Chief Financial Officer 

* - Owns the information 

* -  Able  to proceed 

In addition 

Company secretary

 Despite the name, the role is not a clerical or secretarial one in the usual sense. The company secretary ensures that an organisation complies with relevant legislation and regulation, and keeps board members informed of their legal responsibilities. Company secretaries are the company's named representative on legal documents, and it is their responsibility to ensure that the company and its directors operate within the law. It is also their responsibility to register and communicate with shareholders, to ensure that dividends are paid and to maintain company records, such as lists of directors and shareholders, and annual accounts

The Secretary is Highest Officiating authority in the company  to represent the  company who is appointed as per the Statutory requirements.  He is in frequent touch with the Managing Director and Board of Directors and is an Authorised signatory for Financial Sanctions and Fund releases which involve huge funds. By being the Chief Financial officer with a dual position he can discharge Financial releases very quickly and with an implied authority, if so stated as part of his pre-defined  functions in the resolution at the time of appointment. 

because he is a person who is familier to the compny finance and the position of the revenue and all the matters related to that and in touch withe the managment or drictors to the relevent issues.