Why did only seven of the companions narrate abundantly from the Prophet ﷺ?

There are three primary reasons as to why there were mukthirūn from the ṣaḥābah.

Firstly, all seven of these companions lived many years after the death of the Prophet ﷺ, they became references for all Muslims who came from far and wide to learn the dīn. These new Muslims had questions and those left of the companions recognised the necessity of answering them and not withholding any knowledge. Naturally, then, those companions who lived longer became the teachers of the ummah and their narrations spread further than those who passed away earlier.

Secondly, the place in which a companion resided after the death of the Prophet ﷺ had an impact on their narrations. Companions who remained in Madīna were more likely to be narrated from because it was the hub of knowledge at the time. Likewise, those companions who moved to Kūfa were narrated from more than those who moved to Shām or Egypt.

Finally, the personal circumstances of the companions played a role. After the death of the Prophet ﷺ, Islam spread to places it had never been before. The process of being engaged in these futuḥāt, developing political stability within these new regions and dealing with building new Muslim communities prevented many of the companions from being engaged in teaching.