If you read other top-level comments, you'll find that many people are simply allergic to paying for software. A lot of people don't have cards or even bank accounts so it's just not possible.
It replaced SMS, which was costly, so the deal was pretty clear. Back in those days people were quite aware of SMS prices and 1 EUR/year to replace SMS was a no brainer. It was very popular despite the price. For many people, it was the only app they actually bought.