Microsoft could set minimum hardware requirement that state that mid-range phones should have 1GB and high-end phones should have 2GB of ram. Or they could disapprove apps that require 1GB of ram.
The manufactures are also to blame. For example take the Lumia 720, which is going to be for sale within the next few weeks. They could have released the phone with 1GB ram and it would not be a significant cost increase.
Smartphones with 1GB or ram have been on the market for over 2 years and 2 years is a long time in smartphone world.
Soon developers will start developing games that need a minimum of 1GB ram and a 1280x720 resolution.
The fault is a combination of Microsoft, phone manufacturers and Developers and not just one of them.
In , SoCs RAM is integrated unlike PCs where RAM can be replaced in slots.
Amount of RAM significantly rises lot of cost of an SoC , that's why even for Apple Line-up , iPhones and iPads have 1 GB RAM because they cost more while iPad Mini and iPod Touch have 512 MB RAM to cut down costs.
Not only that, RAM also effects stand-by time and battery drain during Multitasking (more RAM = more apps in background = more battery drain for same amount of time) which means OEM must also increase the battery capacity if they increase RAM.That's why higher end devices with more RAM always have more capacity batteries.