The problem is, they are the same company now, but when all the technology was released, they were 2 very different companies and sirius didn't want XM using sirius' radios or satellite signals and likewise with XM. As a result, they both purposely developed incompatible technology.
Now that they've combined, they don't have enough money to release the proper hardware, so they still use the old equipment.
Also I believe they are NOT the same company in Canada, sirius and XM have to operate separately so there is no sharing of content. Because of this I have an american sirius subscription with the best of XM package so I get both sirius and XM on the sirius radio my dad gave me.