Remember that the Stryker Brigades are/were classified as an "interim force"....they were not initial invasion forces nor were they heavy armour forces....they would fill the gaps.
The Canadian Forces uses the 25 mm armed LAV III in a similar way as the US Army uses the Bradley....an Infantry Section Carrier that can provide limited fire support for dismounted Infantry. The LAV III is the primary Infantry Carrier for the CF.....and is used in the full spectrum of combat related missions....there is no other vehicle in the CF inventory to defer to therefore it needs to be multi-mission capable for many different enemy threats.
The Canadian LAV III replaced the M113A2.....greatly providing more firepower and mobility to keep up with tanks on the battlefield. The Strykers were not developed to replace an existing AFV....they were adapted to many highly mobile roles within the Combat Team spectrum.
The US Army did consider the LAV III and even used it in trials (about 30 of them) in Ft. Lewis in the early 2000s.....as well they trialed the German Luchs and the Italian Centauro.
A good book to read on the development and reasoning for the Stryker Brigade and it ORBAT is Concord's "Journal of Armored Assault & Heliborne Warfare" Volume 2.
You can also read this link.....
http://www.globalsecurity.org/military/agency/army/brigade-ibct.htm 
