Sorry for my late contribution.

Two major reasons: Laos and Cambodia were de jure * neutral states, and LBJ had instituted a bombing halt over North Vietnam the previous October. Nixon could not openly bomb North Vietnam, let alone Cambodia and Laos, without looking like he was ditching negotiations with the North Vietnamese. Nixon had promised (or implied) during his campaign that he would negotiate a Korean style peace with the North Vietnamese, rather than reverting back to Johnson-style escalation.

Pakistan, on the other hand, is an "ally" on the War on Terror. Syria is in an open civil war, in which our opposing side is headed by a dictator who Obama swore publicly needed to step down. Obama, for a variety of complex reasons (drones vs. bombing campaign, domestic politics, 1969 vs 2009 America, etc), also has an easier time politically justifying drone attacks than Nixon did in 1969 with bombing Cambodia, or at least ensuring domestic political indifference to them.

(Now, in practice, of course, Laos and Cambodia were nothing of the sort. Laos had been a de facto North Vietnamese protectorate since the early 1960s, Cambodia's sparsely populated East was largely under North Vietnamese control. Cambodia's ruler, Prince Sihanouk, has no choice but to accept this: the PAVN expelled his officials from the area and basically provided him with an understanding that if he tolerated them, they wouldn't mess with his rule. This was a deal that began to fall apart during the late 1960s as Sihanouk's domestic balancing act began to fall apart, for reasons more related to Cambodian domestic politics than Operation MENU, and would eventually result in the Cambodian Civil War. North Vietnam also launched a major offensive in South Vietnam in February of 1969, and bombing the sanctuaries in Cambodia was one way that Nixon could strike back without ditching the bombing halt over the North. Using the bombing as one of the few viable bargaining chips on the table was probably why it continued all the way up until the outbreak of the civil war in Cambodia, in spite of the ineffectiveness of it.

But the anti-war crowd probably wouldn't have cared about any of this, as Nixon well knew. Logically, if you thought that the Vietnam War was a lost cause and that the United States should simply get out to save lives, you weren't going to be convinced by the tactical rationale of arguments for cleaning out sanctuaries. For context, the hippies weren't exactly being peaceful anymore: there was a time during 1969-1970 where there was a hijacking or an attempted bombing launched by far-left groups every single day over the course of nearly a year. Add in the fact that Nixon was the first President in nearly a century to be assuming office without his party in control of either house in Congress. There was also Nixon's extreme focus on secrecy as a rule in foreign policy, especially when it came to applying bargaining chips with the North Vietnamese. Part of this was the time period, but part of this was simply Nixon. Mel Laird advocated that they be public about the bombing, on the grounds that the people and Congress would eventually find out anyway, but Nixon would have none of it. So, I guess you could say it was a mix of political concern and Nixon's personality. I strongly disagree with Dallek's maxim that history is best understood through the personal lives and psychologies of its major characters, but that is not to say it had no role to play at all.)

Sources:

Richard Reeves: "Alone in the White House".

Pierre Asselin: "A Bitter Peace."

Lien-Hang Nguyen: "Hanoi's War."

James Rosen: "The Strong Man."

Robert Dallek: "Partners in Power."