Hi there,
Thanks for reaching out. Chris is pretty much correct. The limit is due to the bandwidth issues of running those reports for each keyword.
While you may only be interested in the percentage, the report itself uses the SERP and the metrics for each of those pages to generate that percentage. Unfortunately that's a processor intense function so doing so many at once would take a long time and would likely time out.
I hope that answers your question.
Cheers,
Joel.