I am a dual US-Canadian citizen and I wish I wasn't from tax perspective. If you return to Canada, you will continue to have to file and PAY US
taxes for the rest of your life. The filing itself is a pain and you'll have to pay an accountant that specializes in cross-border taxes. Also, you cannot arrange your tax situation to be beneficial in both countries. For example, your RRSP in Canada doesn't count as an IRA in US, and your mortgage interest in US doesn't help your taxes in Canada, and stock options you get in one country won't be favorably taxed in the other, and the US has an alternate minimum tax so often it can actually be higher tax rate in US (i.e. you live in Canada and do RRSPs and RESPs etc to lower Canadian taxes, but then get dinged by US).
Another way to say it is you will always pay whichever is higher and surprisingly that can be the US. When I've lived in Canada I've often had to pay $40k or more per year to US on top of all my Canadian taxes!
Another part of the taxes which gets tricky is residency. For example, if you live in Canada for a while then change residency to US, you have to be careful because Canada will consider you leaving and then all assets (like house you own in Canada) become taxable as if you sold it (even if you didn't).
However, if you don't sever residency then you will get double-dinged by state taxes because the individual states don't have any tax treaty with Canada.
Furthermore, you have to report your maximum monthly balance in all world-wide accounts to the IRS for the rest of your life. Not only is this a pain, but feels like invasion of privacy.
And lastly, don't think you can just renounce your US citizenship later on because when you do that they will tax you on your world-wide assets! You have to pay like 15% of your net worth to give up your US citizenship!
Don't get me wrong, there is benefit of being dual that I enjoy (mainly related to employment opportunities). But it is a serious, serious tax hassle.
Anyway, that would be the primary reason I would think people don't get the dual citizenship.
I WOULD advise getting US citizenship if you planned to permanently sever your residency in Canada. But if you have chance of going back, or going back and forth (like I do), be very careful.