Siakim was good for like 15 games to start the season. That really seems like a bit of anti-Boston bias that he got in over Tatum.
Yeah Siakam got some benefit to his early season outburst. He looked like a top 10 player and maybe even a fringe MVP candidate (making the top 5 ballot rather than winning) during that period.
That clearly stuck out so strongly in many voter's minds that they missed how much Siakam trailed off as the season went on. He was more of a borderline call for the 3rd team (maybe top 20 rather than top 15) than a top 10 player by season's end.
^this, while Tatum went from a top 30ish player to borderline top 15 as the season went on.