Monorails are, in general, not at the top of the list of municipalities for public transport.
They have less capacity than metros, more expensive than trams and may make citizens unhappy by closing up views of cities. Thus the total monorail projects in the world is not much, just above 300km. The most famous one is, of course, Shanghai Maglev Train, the fastest commercial train service in the world with 431 km/h.
Low cost compared with metros is one of the main reasons for choosing monorail. Libadiye Monarail is a good example for this, with 25m TL/km cost, which is one fourth of the cheapest metro line in Istanbul. On the other hand, Sefakoy-Basaksehir monorail will cost almost like a metro. Total budget for 15 km monorail is 1440m TL (96m TL/km). This unit cost is very similar to Uskudar-Umraniye-Cekmekoy metro which is under construction (103m TL/km).
Trams, which are widely chosen in lighter traffic in many cities, is not a prefered solution in narrow streets of Istanbul which are already full of cars.
Hills of Istanbul is another reason for monorails. Metros become even more expensive in such cases. Chinese city Chongqing which has the longest monorail network in the world may be a good example for hilly areas.
Most of the monorail projects in Asian side of Istanbul are planned as a solution to main transportation problem of that area. All main transportation routes are in west-east direction. Moving in north-south direction with public transport is almost impossible. Libadiye, Tuzla, Kartal, Maltepe and Atasehir monorails are good examples for connecting main transport routes to each other.
Two projects in European side, Sisli-Beyoglu and Levent-4.Levent monorails, on the other hand, is mainly going through hilly areas with dense population.
Two others, Sefakoy-Ataturk Airport and Sefakoy-Halkali are projects to connect passenger terminals (airport, railway) to main transport lines.
Last monorail project is Sabiha Gokcen Airport-Formula monorail, which connects Istanbul Park (race circuit) to public transportation system via airport.