Tories Not International Statesmen.
While Conservative MP Dean Del Mastro claims that the reason for him not going is personal safety, his leader's actions to this point would seem to indicate that Tory MP's can't say or do anything without permission from the PMO. This means that, despite the denials, the PMO probably ordered Del Mastro to not go. Since the purpose of the trip is to get the Arab side of the conflict to have a better picture of the problems in the Mid-East, I can see why the PMO would object.
Stephen Harper has shown that he is squarely on Israel's side in the conflict. Harper, who apparently has the conservative problem of being unable to see other peoples' point of view, would not want any of his MP's "tainted" with knowledge that might contradict his world view. Combined with his constant micromanagement of government Harper's has shown it's not unreasonable to assume that Del Mastro's marching orders (or lack of marching as the case may be) came straight from the PMO.
This of course speaks volumes about the lack of international savvy that the current Conservative government has. By picking sides in the Middle East, especially the side of Israel, the Tories have done more damage to Canada's reputation internationally in a few months than 14 or so years of Liberal inaction ever did. Of course the current US administration likes having a toady in government here and I suspect that what George W. Bush thinks is all that Harper cares about. The sell-out of Canadian interests to the US is well under way with this government.
Of course this will cause Harper problems here at home where there is a large Arab-Canadian community. This community is largely based in Toronto and Montreal, both cities Harper needs to start voting Conservative if he wants a majority government next time around (or at the least stay in government). With his actions, and those of his party, he is doing his best to turn those votes away and when nationalism is involved, voters have long memories.