Why writing about experimental results is difficult?
It's difficult to write about other people's work because:


It feels redundant repeating the same introduction that appears in all other papers over and over again.
Oftentimes it's impossible to get an in-depth understanding of other people's work, but yet you're expected to describe their work as if you fully understand it.
The way other people explain their work in writing can be difficult to understand because of terminology, background or perception differences.


It's difficult to write about your own work because:


Quickly after you start writing, you realize that there are so many more experiments you could have done to confirm your results, and that your readers may expect you would have conducted in the first place.
Explaining your results is easy only when they match existing theory. When you have unexplainable results, you actually need to come up with an explanation (= theory), which is not easy.
The way you end up ordering your text only rarely matches the actual chronological order of the work you've done. Creating this somewhat fake narrative may feel dishonest and demotivating.
A typical pattern in presenting information is hierarchical organization into mutually exclusive abstract groups. This could be difficult to do, simply because the world is very complicated.