#1 Reason: People mention "the streak", but in those years, the SAG winner won other big awards as well.
2015 - Leonardo Dicaprio won the BAFTA, Globe, Critics Choice, and Satellite
2014 - Eddie Redmayne won the BAFTA and Globe
2013 - Matthew McConaughey won the Globe, Critics Choice, and Satellite
2012 - Daniel Day-Lewis won the Critics Choice, Globe, and BAFTA
2011 - Jean Dujardin won the Globe and BAFTA
2010 - Colin Firth won the Globe, BAFTA, Critics Choice, and Satellite
2009 - Jeff Bridges won the Globe and Critics Choice
2008 - Sean Penn won the Critics Choice. Rourke won the Globe and BAFTA.
2007 - Daniel Day-Lewis won the Globe, BAFTA, and Critics Choice
2006 - Forrest Whittaker won the BAFTA, Globe, Critics Choice, and Satellite
2005 - Phillip Seymore Hoffman won the BAFTA, Globe, Critics Choice, and Satellite
2004 - Jamie Foxx won the BAFTA, Globe, Critics Choice, and Satellite
2003 - Johnny Depp just the SAG. Penn and Murray won the other big awards.
#2 Reason: Never got a BAFTA nomination.
Since 2000, only McConaughey and Washington have won Oscars without a BAFTA nomination, and in both those seasons, their films had problems getting in their screeners on time.
#3 Reason: Affleck won the BAFTA, Critics Choice, Globe, and 32/36 critics circle prizes.
Much like the other Best Actor winners, he won the majority of the big awards. If you want to talk about streaks, Brody was the last time a Best Actor winner won without winning the most big awards.
This is why I am going with Affleck.