I picked it mainly for 3 reasons. In my opinion,
1. _ kind of hide itself & give more prominence to the rest of the code (logic) 2. If we pick a long name like CalDOM, it becomes a PITA when it repeats so many times. 3. Not to be conflicted with jQuery by using $
To avoid conflicts, CalDOM provides an official workaround to define the alias before loading: https://caldom.org/docs/#_cal_dom_alias
If you don't like the underscore, there's an official way to alias it: https://caldom.org/docs/#_cal_dom_alias
I actually forgot to mention this in the documentation, your feedback prompted me to do so.
Rest is just the nature of JavaScript my friend. I hear you on Python.
For the latter Python encourages slug_case_naming_schemes and _ is a valid identifier in Python too. So there's nothing going on here beyond normal Javascript syntax which can be closure heavy -- a main feature of Javascript.