Here's why:
* Comparative: "Soon" already implies a relative sense of time. We don't usually say things like "sooner than soon" because "soon" already implies a timeframe that is relatively close.
* Superlative: Similarly, we don't say things like "soonest" because "soon" already implies a time that is close to the present.
To express different degrees of proximity in time, we use phrases like:
* Very soon
* Shortly
* In the near future
* Before long
These phrases convey the desired meaning without using comparative or superlative forms.