Both.
It also started out in the early 90's and was pretty much the only option for high resolution imaging.
Adobe has just kept up with user demand over the years.
Especially in business and educational environments.
Since Adobe products are so commonly used in education, it makes sense from a business perspective to stick with it. And vice versa. Students want to maximize their employment and earning potential when they are paying good money for the education.
Which is the simplified explanation of "industry standard".
It doesn't come down to only better marketing or better product.
Situations are different. Maybe your needs don't justify the cost of Photoshop, especially if you aren't entitled to any discounts or upgrade prices.
Then you're in the market for a more affordable solution, which changes the whole scenario.
If someone just wants to play around with their digital photos, then they don't have a need to pay for something like Photoshop, which is why other products are made.
Since most other lower and mid-range image editors are modeled after Photoshop especially in terms of interface design, it's safe to assume you could grasp a lower level app if trained with Photoshop also.
I'm not saying that you can't achieve the same results with another app.
I'm just saying that these are the reasons you see most people discussing Photoshop instead of PaintShop.
If it works for you, then great stick with it.
But the original topic was "why so much Photoshop and not Paintshop".