There has been - implies something is still happenning. There was/were - implies that whatever was happenning has now stopped. In your example you should say "there was", not "there were", because the dramatic fall is singular not plural.

L'emploi de was/were (to be past simple) fait référence à un moment précis dans le passé (yesterday, last week, last october etc. même si le locuteur ne l'indique pas c'est implicite ) cela veut dire que l'action est terminée, achevée dans le passé. has been 'to be present perfect) sous entend: jusqu’à présent cela veut dire que l'action persiste encore ou a des répercussions sur le présent. Remarque: Le present perfect n'existe pas en français pour mieux le comprendre on utilise des adverbes temporels pour situer l'interlocuteur dans le temps par exemple ( recently, this morning, today, this years etc.)

"There has been..." tells us about the present. "There was/were..." tells us about the past (and only the past). "There has been a dramatic fall in the sales of chocolate" = and the effects of this can be felt now. "There was a dramatic fall in the sales of chocolate..." (you need to tell us when this happened) = this happened in the past. It's done and finished. It doesn't tell us of any effect on the present.