To understand why these questions are problematic, you first have to understand the goal of SU: To accumulate a vast store of knowledge that will be helpful for future visitors who are encountering issues with computer software and hardware.
It's the helpful for future visitors that most of these questions fall short on - Software recommendations generally need to be very specific to the person asking the question's needs in order to be useful to him or her, and this inherently makes the answers less useful for others who might have a similar, but ultimately different set of requirements.
The other part of the issue is the future part. While questions can be edited and new answers added, we generally want to avoid answers that will inherently outdate themselves and become wrong. A particular piece of software may not evolve quickly, but new software enters the playing field all the time. A year is a long time in the software world. I myself have been here for more than 4 years - Any software I recommended 4 years ago would be very well outdated and useless for anyone today (unless the requirements from the question were exceedingly strict). It's far too much work to keep software recommendations updated and accurate.
The other thing you have to keep in mind is the software recommendation questions that you do see on this site are not strictly representative of what there would be if we changed this rule and allowed software-rec questions. There are some software recommendation questions that can be phrased well enough, but for each there are also many, many thousands of questions that are absolutely terrible, and would just be noise and clutter. Because we provide other means to help with this (Come visit us in the super user chat and ask instead!), it's much healthier for the website to simply mark software recommendations as off-limits.