Well, I'm no legal guru (or, at least I don't play one on TV)... but my take on it is this:
Legally, the first burden of the plaintiffs is to establish/demonstrate that BC is an unfit father. Until that's established, it's pointless to talk about where the girls might have a "better" home.
Until the plaintiffs can demonstrate to the court, that BC is an unfit father, then the girls are "by definition" best served by being with their father (in the eyes of the court).
In other words, the defense doesn't want the waters to be muddied by testimony like "in Canada the girls would have a better life, a more stable life, etc" discussed (yet). These things are irrelevant unless it is first established BC is "unfit".
Another way to look at it (very hypothetical) Let's say someone wanted to come in and take your kids, let's say they were really wealthy, and could provide for them a "better" home, meet all their needs, and shower them with love. Even if it that were true that the (hypothetical) plaintiffs in this example could demonstrate that they could provide a "better" home than you... it's pointless legally, unless it's been established that you are unfit. [ Again, this is an oversimplified example, but kinda explains the rationale of the law ]
In the BC case, the defense is trying to limit the scope to "first things first". This way they can focus their defense. If the plaintiffs have evidence that BC is unfit father, then it's a non-issue. If they don't, then it will be a short day in court. [ Obviously the plaintiffs push/request for a psycho exam is along the lines of trying to establish "fitness" as a parent ]