There are several reasons why Pakistani soldiers prefer serving overseas on peacekeeping missions.

The short answer is "More Money, Less Danger." For peacekeeping duty, the pay is several times what troops make while in Pakistan. Second, it is safer. There are 90 fatalities per 100,000 Pakistani peacekeepers per year. It's more than double that for the larger number of Pakistani troops serving in the tribal territories for the last four years. While only about 10,000 Pakistani troops a year get to serve as peacekeepers, over 100,000 a year have been serving in the tribal territories along the Afghan border. Oddly enough, this part of the world has been producing mercenary soldiers for centuries.

The UN's peacekeeping army of 120,000 troops, would be much less effective, were not for Pakistani, and other troops from South Asia. About a third of UN peacekeepers come from nations that were formerly part of the British Empire (mostly from what used to be British India; India, Pakistan, Bangladesh, Nepal.) To maintain and protect is empire, the British recruited lots of local troops, and trained them as they would British soldiers. While most of the officers were British, most of the NCOs were locals. Before the British relinquished its colonies after World War II, it also trained many locals as officers. Much of this Western training and military traditions took hold, especially in South Asia.

The British encountered some formidable local armies when they moved into India three centuries ago. But the British had better technology and more advanced military doctrine. The Indians noted this and some of the local rulers quickly, but not quickly enough, adopted the superior British practices. When the British left, the South Asian armies remained very British in the way they trained and operated. That meant well trained and well led troops, but without all the gadgets that Western nations lavish on their soldiers. For peacekeeping, the disciplined and reliable South Asian soldiers are excellent peacekeepers. Those from other less affluent nations often lack the discipline and good leadership, and account for most of the peacekeeping scandals.

Meanwhile, corruption, casualties and lack of success are discouraging countries from contributing their troops for peacekeeping. The corruption angle is interesting, as it pertains both to the corruption within the UN bureaucracy, and the corrupt atmosphere the peacekeepers operate in and often succumb to. Casualties are expected, but the contributing countries feel a lot of their troop losses are the result of restrictive UN rules that limit what peacekeepers can do. This, in turn, is believed most responsible for a lack of success for the peacekeeping missions.
India, Bangladesh and Pakistan are not happy with the lack of volunteers from other major nations. The chief reasons for that are the same ones annoying the current peacekeepers (corruption and restrictive rules of engagement). In addition, the major military powers (with the exception of China and Russia) feel they already contribute quite a lot in the form of money to pay the peacekeepers and need not contribute troops as well. And the contributors are also upset at the lack of results.