Why isn't QucsStudio used more often in place of ADS?

Don't know about particular differences but if one software is available free to college students, they'll tend to stick with it in their careers. If ADS made this possible, it could explain a lot.

Speaking for myself only, I've seen QUCS since the 1990's among other choices available of electronic circuit simulators (even the shareware versions). QUCS seemed neither better nor worse. These days what makes a simulator stand out could be whether it supports numerous IC's in popular use, and is kept up-to-date by its software team. Being in the Spice family seems important as well.

Knowing ADS (self guided or training) and having extensive support for ADS is certainly one reason. You didn't mention tool feature limitations. ADS has more comprehensive modelling capabilities, and a complete and smooth workflow.

I had tried QucsStudio when my ADS license had expired, but was very happy to return to ADS after a few weeks because I found it difficult to get my work done in QucsStudio. I guess many tasks could be completed somehow using QucsStudio, if I would spend more time to become more familiar with the tool, but "time is money" so ADS might be the cheaper solution overall.

In a nutshell: Momentum. There is an RF solver for Qucs but it is not on the same level nor as user-friendly. ADS also has cosimulation capabilities with MATLAB. The fact that Qucs-S uses engines like Xyce or NGSpice is a huge asset and makes it superior to QucsStudio, however, a big downside is that the optimizer in both Qucs-S and QucsStudio only works with Qucsator which uses its own custom models, so that means you can't use non-linear transistor models from Qorvo or Macom or whatever and optimize your design. You can also optimize your design directly on Momentum, something that I haven't been able to do with Qucs. The Qucs optimizer is sub-par, at least in its versatility and user interface, with respect to ADS.

If you are only working with S-parameter files and transmission lines, Qucs might be sufficient, but when you start working with nonlinear models and so forth, Qucs falls short as to what can be done.

Also, Qucs is not as user-friendly as ADS, nor does it provide the technical support that Keysight (Pathwave) provides. In fact, the documentation in Qucs is a nightmare, it will barely explain the basics and most things are "left as an exercise to the reader". Updates are not as frequent and it is being offered with no warranties, which means that you can't call them and demand anything from them. Momentum has a reputation of providing very accurate, consistent and dependable results. Again, when was the last time QucsStudio was updated? 2020? I don't even know if QucsStudio is still being maintained, it seems like an abandoned project to me. Qucs-S is updated periodically, but still, the problem remains that it is open-source and you either take it as it is or you don't.

Finally, Qucs doesn't enjoy the level of reputation that ADS does, so publishing in high-impact journals might not be as easy if you are using Qucs.

There are other things in which I believe ADS is superior, but the ones I mentioned are, perhaps, some of the most important ones. That is not to say that Qucs-S or QucsStudio are bad, I think they are wonderful tools, especially considering that they are completely free.