Because fuels have changed and are not what they used to be. That's the simple answer. You need to have the right tools and the right information to deal with the fact that today’s stored fuels are nothing like yesterday’s stored fuels.
Today’s diesel fuels are significantly more prone to developing microbe problems than stored fuels of decades past. The removal of sulfur and the low caps on aromatic content have created a fuel that burns cleaner, but which has very little resistance to microbial growth (both sulfur and aromatic molecules tend to inhibit microbial growth). Today’s diesel fuels attract and hold more water. They’re also made with cracked petroleum feedstocks that weren’t as common back in the day.
All of these changes, put together, mean today’s diesel fuels are more prone to filter plugging, corrosion, engine deposit formation, and losing their stability in a much shorter period of time than the fuels of yesterday.
Gone are the days when taking care of stored fuel meant just setting it and forgetting it. These fuels need proper condition monitoring, to stay ahead of the kind of fuel changes that aren’t a matter of if, they’re a matter of when.
Condition monitoring can be as simple as pulling samples monthly and looking to see if they look (or smell different). Yet not everyone even does that. There are other, better things you can do as well. Periodic microbial testing. Monitoring the buildup of water in the tank and removing it regularly.
There's a lot more that can be said on the subject. But it's important to have a good grasp of condition monitoring. If that challenge seems like more than you can put on your plate, find a knowledgeable partner that both knows what to do and is willing to let you follow along in the process with them.