First thing first, battery. Dual sim receptors requires more power to manage their resources. It drains battery much faster than single sim phones, unless we have phones that have >3000–4000 mAh batteries (which will eventually degrade their performance after a year).
It confuses. We have WiFi/Bluetooth, Screen rotation, Mobile data, GPS, NFC, Flight mode, hand-glove mode, DND switches, that are already kept in cluttered way, like a electricity switch board. When it comes to dual-sim phones, additional switches get introduced, individual Sim 1/Sim 2 notifications get introduced in the same UI. This is okay with some of us, but not everybody. Although, I accept dual sims are useful, but I personally prefer having 2 sims in 2 phones rather than having 2 sims in 1 phone.
Steve wanted to make iPhone the simplest phone in the market. “His vision for Apple was a company that turned powerful technology into tools that were easy to use, tools that would help people realize their dreams and change the world for the better,” Cook said of Jobs during his commencement speech at George Washington University.
Third and very important point from engineering perspective – Apple pays too much attention to details and quality. Adding dual sim support eventually downgrades the performance and the hardware build quality. In my opinion, Nexus phones for Androids are as beautiful as iPhones for iOS – they both are benchmarks and have single sim slot.
Apple sells relatively expensive products primarily to developed countries (and developed enclaves of developing countries, like urban China). Dual SIM demand is primarily from developing countries with much poorer populations.
Apple could make a separate dual SIM model and sell it to developing countries, but they generally try to make as few models as possible. This simplifies the supply chains, allowing them to do things like deliver 13 million phones with a never-sold-before design in a single weekend. It took them years to get to the point where they could sell one model of phone (at a given size) anywhere in the world; they don’t want to give that up.
Apple could add this feature to their single model, so that all phones are dual SIM, but they would either have to make the phones thicker, or remove something that’s already inside them, like part of the battery or the high-quality Taptic Engine vibration motor. Basically, they’d have to make the phone worse for their core markets.
In short, in order to sell iPhones to customers who demand dual SIM phones, Apple would have to give up something that’s more valuable to them. It’s just not likely to happen.
What’s more likely to happen, though, is that Apple will introduce some sort of programmable SIM that gives the iPhone the features of a dual SIM phone without actually setting aside physical space for a second SIM tray. Apple already uses a version of this in cellular iPads in some countries, which can be switched between participating carriers simply by visiting the Settings app.