I don't know :-)

maybe cause of this ?

http://noknok.tv/2011/06/06/nokia-bo...-3-days-to-go/

as well as look at subject discussion
"The future of devices: tablet, smartphone and OS wars"

plus i think nokia connection event will then be used to clearly announce and discuss windows phone and nokia strategies.so as to not avoid conflict between the device release and strategy announcement
:-)