I can think of a couple of reasons why NASCAR won't bring the Cup cars to Montreal yet, but there's also room to question those reasons.
1 (Most obvious). They don't really want to remove an oval race from the schedule to add another road course race just yet. This is perhaps in large part because it's less of an immediate moneymaker, given the comparatively small number of fans that can attend a road course race.
That said, I'd argue that having at least a couple more road course races makes long-term sense, because it sure seems like these races get talked about a lot, increasing the exposure of the sport to non-fans ("Hey, look at SportsCenter! These guys don't just drive in circles! And this mayhem is kind of fun to watch!"). It may also help to nebulously improve the overall "quality" of the series, breaking up some of the oval monotony and broadening the range of skills a driver needs to be successful. Among the kind of NASCAR fan who visits nascar.com, there's overwhelming support (over 75% if I remember a poll on the site correctly) for adding more road course races.
I suppose the counterargument to this is that the kind of fan who doesn't go online and vote in a poll like this is probably one who hates having road course races in NASCAR. And even if a pretty large majority of fans want to see more races, I figure that NASCAR calculates that the number of fans they gain or retain by diversifying the track selection is smaller than the number they'd lose. In other words, even if they're the majority, most people who want to see more road courses will nevertheless keep watching, but the minority who hates them might object much more strongly and stop watching altogether. This may or may not be correct reasoning on NASCAR's part.
2. They don't seem to want to let their flagship series leave the country, at least not for a race that matters. I imagine that this is because they're worried about offending their traditional core of fans, the sort who want the series to remain as purely American as possible.
Here, too, it's possible to raise an objection - that there are quite a few people outside of the US who could become stock car racing fans, and that racing internationally would significantly improve NASCAR's image among the rest of the general public. Plus, the group of Southern hicks who would stop watching NASCAR because it became too international is probably pretty small at this point. But it's awfully hard to know, and I'm willing to give them the benefit of the doubt on this one.
So, I think bringing the Cup series to Montreal really would be risky, and NASCAR isn't an organization that likes to take a lot of risks. But the potential rewards are also quite large. I just don't think we know enough about fan loyalty to say with certainty to say that they're making a bad decision. Would I like to see the Cup series race outside the country more often? Yes, absolutely. Would I like to see more road course races? Unquestionably. But I can't speak for the silent majority of fans.
Congratulations, by the way, to Piquet. I'm really happy to see him get a Truck win, because from everything I know, he's an extremely classy racer. He deserves for a bit of good luck to come his way. And I enjoy seeing foreign-born drivers have success in NASCAR.
