You might think that JR East has installed Suica gates in every station but this is not the case: as of 2018 Suica is installed in roughly half of JR East’s 1667 stations. The reason is cost. Unmanned stations have simple Suica gate readers but apparently the cost of hard wiring these to the Suica data center is an obstacle. Fast local processing is one of the advantages of Suica but the dedicated network backbone costs for linking and syncing with JR East servers doesn’t come cheap.
JR East is fixing the cost problem by developing a new cloud based Suica gate reader that can easily be installed anywhere. The trade off is slightly slower speeds, perhaps, with the benefit of lower installation and maintenance costs. JR East said they expect to reach 100% Suica deployment with the new model in the 2021 fiscal year (Super Suica year) and hope to sell it internationally.
April 2021 UPDATE: JR East finally announced cloud based Suica and extended coverage for the Tohoku region going online with 44 stations in early 2023, around the same time that Mobile ICOCA launches. This same cloud system is expected to drive JR East closed loop ticketing and more mobile based services.