Andrew Sullivan quotes at length from an anonymous reader who has very interesting thoughts on the Christmas underpants terrorist. You can read Sullivan's post here, but I will attempt to summarize.
It starts with the question: "Why didn't the terrorist just go to the airplane toilet and blow up the plane from there?"
And the suggested answer is that al Qaeda probably had a good idea the attack wouldn't necessarily result in carnage – i.e., they suspected that this naive young recent-convert-to-fundamentalist-Islam might get caught at airport screening or something like that. The goal of terrorism, one needs to remember, isn't necessarily to kill, but to create terror — something at which (arguably) the Christmas underpants terrorist was successful (gauging the reactions — or over-reactions — of some in the political sphere).
So that's probably why he wasn't trained to go to the airplane loo to detonate himself. Because that wasn't necessary in order for al Qaeda to spread fear. It was to be done in the open. If the bomb detonates, great (from an al Qaeda perspective). If it doesn't, you still get nationwide fear and over-reaction, something you wouldn't get if all that happened was that the Christmas underpants terrorist took a humiliating stroll from the airline toilet back to his seat with a failed bomb in his pants.