The גמרא in שבת פח. says that at the time of מתן תורה, ד' forced כלל ישראל to accept the תורה, "כפה עליהם הר כגיגית". ד' suspended הר סיני above the Jews and threatened them with being crushed unless they agreed to take the תורה.
The מדרש תנחומא (נח:ג') asks: כלל ישראל had previously proclaimed "נעשה ונשמע", and thereby voluntarily agreed to accept the תורה. So why was it necessary to force them with "כפה עליהם הר כגיגית"?
The מדרש explains that the קבלה of "נעשה ונשמע" was limited to "תורה שבכתב", the written תורה. However regarding "תורה שבעל פה", the oral תורה, כלל ישראל refused to accept it. The מדרש goes on to say that owing to the tremendous difficulty involved in learning "תורה שבעל פה", כלל ישראל was unwilling to assume this task. Therefore, ד' had to compel them with "כפה עליהם הר כגיגית".
The גמרא in שבת states that despite their initial refusal, after the events of פורים, כלל ישראל decided to accept even תורה שבעל פה willingly. The גמרא derives this from the double לשון in מגילת אסתר- "קימו וקבלו", i.e. they “ratified and accepted”. However, the גמרא does not explain what about the salvation of פורים, gave כלל ישראל the ability to overcome their original concern about the arduous nature of studying תורה שבעל פה. Indeed the חדושי הרמב"ן on שבת פח. raises this point by observing that בנ"י had experienced miraculous salvation by יציאת מצרים, and yet that did not provide them with sufficient impetus to accept תורה שבעל פה. So what about פורים was different?
In order to answer this question, we need to reflect on what המן had decreed on the Jews. The גמרא in מגילה טז. comments on the words in מגילת אסתר "ליהודים היתה אורה" that "אורה" refers to תורה. רש"י there explains that the גמרא means that המן made a גזירה forbidding the study of תורה. This was the first time in Jewish history that learning תורה was forbidden by law.
With this background in mind, we can suggest an answer as why the בנ"י became motivated to accept תורה שבעל פה after the deliverance of Purim. Haman's gezeira against לימוד תורה gave כלל ישראל the opportunity to experience life without the learning of תורה שבעל פה that they had become accustomed to over the last 700 years. כלל ישראל was now able to contrast life with and without תורה שבעל פה. Their conclusion was that the geshmak and depth of learning תורה שבעל פה, by far outweighed the strenuous effort required to properly comprehend it. They realized how wrong it had been to not want תורה שבעל פה when ד' originally offered it to them at מתן תורה. As a result, כלל ישראל dearly longed for תורה שבעל פה, which המן had taken away from them with his גזירה. When המן was killed and the Jews were saved from המן’s גזירות, כלל ישראל demonstrated their jubilation by voluntarily accepting תורה שבעל פה.
