Satellite collects extremely large amounts of data, with some of the most recent high-resolution optical and radar satellites capturing gigabits of information per second. However, “downlinks”, the acts of sending the data back to Earth, are limited by the current technology. Downlinks can only happen whenever the satellites are within reach of the ground stations sensors, which happens a few times per day on sun-synchronous orbits, with each downlink session lasting 9 minutes at maximum and limited to some hundred megabits per second. That's because sun-synchronous satellites travel at speeds ranging from ~7.1 km to ~7.8 km per second (4.4 to 4.8 mi/s).
Which means satellite do not continuously record data because they may not be able to send it all back down. This is one of the many reasons why data for a specific time and location might not be available, even if satellites were passing over that location at the time. Clouds could be another really good reason, as the Earth is 70-80% covered by clouds on most days. As for SAR satellites, while they can "see through clouds", emitting radio waves is energy consuming and they often need to stop emitting and recharge. That’s what their solar panels are for.
Waiting until the satellite passes over a downlink station also means that it could take several hours for captured data to be transmitted back down to Earth, then additional time is necessary to decipher it and transform it into usable data. Some of the processing that is required includes adding metadata to the file, such as exact geographical location, sun elevation, atmosphere measurements, and many other factors that are crucial for geospatial experts to make sense of the data they received.
However, as technology improves, the time between capture and final delivery to the end-user shortens. With AWS building ground stations next to their facilities, they were able to receive and upload test satellite data into AWS servers in less than 1 minute. Tightening data delivery time will eventually become crucial for many satellite data applications relying on ‘real-time’ updates, such as disaster management and incident responses.
Source: Image of the Svalbard Satellite Station illuminated by the Northern Lights. The Svalbard Satellite Station is one of the only two Satellite stations close enough to the pole to be able to see low altitude polar-orbiting satellites on every revolution as the Earth rotates.
Image by Reuben Wu, shared by Lonely Planet.