You could argue it's been done because
a) Tom and Claudia were on. Two guests who fall (ironically given their sports!) into the 'young sports person' category
b) Putting them on the One Show also gets an audience who wouldn't normally consider watching the main SPOTY programme - so an additional promotion within the BBC One family.
On a side issue. Without wishing to decend into a Daily Heil anti-BBC rant, but we had the BBC Music Awards on Monday, and we have the SPOTY on Sunday.
Wouldn't it be more cost effective for BBC Events to have the Music Awards on Friday, then reconfigure the stage on the Saturday and do the Sports Personality bash from the same venue on the Sunday evening?
Same arena, same staging crew, same OBU crews, same broadcaster.
My user name might look like Hatton Cross, but it's pronounced Throatwobbler Mangrove.