Dear Sahil,
I don't think this is the right forum for this question: P21 did not develop SDTM nor was involved in it.
As this is a question about "why this decision was made as it is when SDTM was originally developed", the much better forum is the "SDTM experts" forum on LinkedIn: https://www.linkedin.com/groups/2758697/
Most of the people who actually developed SDTM are members of this LinkedIn group. You won't find them on this forum...
Why RFSTDTC, RFXTSDTC, RFENDTC, RFXENDTC, RFICDTC & RFPENDTC are called as record qualifiers in DM dataset, whereas --STDTC variables in other domains are called as timing variables ?