Neither. It is deliberate.
It’s not difficult and the developers are interested in it so you deliberately don’t implement it… ?
Vertical zoom is very useful when dealing with content that has a large dynamic range. For example some audio can be really quiet which makes it difficult to see details and make precise cuts.
Here is a video demonstrating making a cut in Ardour and then again in Reaper (with vertical zoom). You can see instantly that it is much easier in Reaper because I can see what I’m doing. Now if this was a one off kind of thing it’s not too bad to roughly cut the audio, boost the gain, then make a more precise cut and reduce the gain again, but doing this hundreds of times is not practical.
Here you can see a section of audio from just 3 of the channels. In Ardour (top image) I’m in logarithmic view and the waveform looks like noise. In Reaper (bottom image, with vertical zoom) you can see more detail and see that there is some audio there as well as noise.
My primary use is cutting samples. Mixing is not so important (although I use busses when needed). Only having to deal with one track makes my life much easier, I can cut and move hundreds of regions really quickly and I can apply batch scripts which work more quickly on a single track than multiple. But the main reason why it is necessary for me to use a single track is automation. I need to apply pitch shifting (and sometimes other automation) to each channel equally and this can only be done on a single track. It would be far too laborious to copy the automation of thousands of regions between 12 tracks and make sure any future edits are in sync.