why goodwill account is debited while issuing shares to promoters ?

The company may issue shares to its promoters because they already provided their services while incorporating the company. They have helped in formation of the company, starting it and even setting it in a position to do its business.

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The Goodwill A/c is debited because it is assumed that activities (and all the efforts) of the promoters have resulted in making the company profitable. So, their activities and hard-work have given the company clear and distinguished advantage which it can use (after formation) as a base on which it could start ad carry on its business.

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So, it cannot be said that the company has started from "zero" level. Instead the promoters have already set the necessary tools for the company to function and grow, well before the time the company has formed.

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Hence, shares may be issued to promoters for such services rendered by them. The benefit of this service will be there at all times during the normal life time of the company. It is appropriate to debit Goodwill account and not any expense account for passing journal entry for this purpose.

[Note that promoters do not need to pay for this because they have already provided the services.]