Speaking generally, there are four main reasons, and two implied reasons, why nations prefer highways to railways. I would assume these hold in Pakistan as well as anywhere else.
- Ease of interface with existing infrastructure: Railroads are specialized high-efficiency, high-mass networks that excel at getting ton-miles on the cheap. They require, however, extensive interface infrastructure to be of use: cargo and passenger terminals, secondary city planning, parking garages, container yards, and local public transit. Highways, by comparison, are directly plug-and-play with local streets and roads, no additional infrastructure is required, and the rolling stock fleet that already exists can be put directly to use on the highway as readily as it is in use on local streets.
- Network externalities: for the reason above, highways are of immediate utility to everyone who owns a car in the country, whereas railroads are only of use to those people whom the network directly serves. If I want to use a train, I can only go where the train goes. If I want to use a car, I am much less limited in where I can go.
- Strategic value: In truly massive territories the railroad shines as a means of military logistics, but it's also vulnerable and once a link is broken there are usually not many redundancies available. The same military vehicles that gain utility from roads, however, can also operate cross-country if need be. This makes highways a very flexible logistical resource for your military in times of national defense, compared to railroads. Pakistan, in particular, doesn't have much distance from it's likely strategic threats so a railroad network is likely to be swiftly disrupted.
- The oil industrial sector has a strong lobby in favor of highways because they dramatically increase demand for gasoline and diesel fuel, essentially creating a large consumer sector for fuel. Railroads also sometimes buy fuel, but nowhere near to the degree that large fleets of passenger cars and tractor-trailers do.
The implied reasons that knock on from this are:
- Technological lock-in: Once you have a road network of any kind, highways interconnect more readily than do railroads. The more you build highways, the more this becomes true. Since roads date back to ancient antiquity, they've got a major head-start vs. steel rails, no matter the technical advantages of rail.
- Politically speaking, people feel the benefit of the highway system more directly, and so are more likely to consider it a success, even at lower levels of effort.