I'll start off with the blanket reason.
Why on earth would anyone buy a Vauxhall?
Every other brand has a certain quality. For example reliability, handling, design flair, cheapness
Vauxhall has none of this. They're just a big steaming pile of mediocrity bodged together by a US firm which relies on government handouts.
This is not the 80s when they were the rep mobile of choice. If a rep turned up in a Vauxhall you'd wonder if the firm would still be in business in six months.
The Vauxhall Corsa exists purely because people want a small car but their budget doesn't extend to a reliable Japanese brand.
Now let's get onto the Mokka. I'm not going to make a big deal of the styling which is just bland, you either like it or you don't. What I'm going to poke at is the fact that they've taken a small hatchback and inflated it. The car looks like it should be a motorway cruiser but it isn't, the engine is noisy and the whole experience is noisy.
It has less boot space (200L less) than a Peugot 308 which is a small hatchback.
The engine is literally not designed for the car, GM pulled out of the parts bin an engine designed for a subcompact to tempt the Euro market with economy and the power figure listed is only at the top of the rev range. So you're revving the bollocks off this engine and killing fuel economy in the hope that the car might not be slower than a drugged up sloth.