While it's true that just over a hundred people made at least one transaction in Hackagotchi, and that some people played it for hours a day, I like to think of its success in terms of how it took a small, close-knit community and made it more approachable, and how people who joined to play this open-ended game are still active in the community years later, long after it had been discontinued.
And before I forget ... egg hatch wen?
With Hackagotchi, you can have a farm and cute adorable Orphei that earn you passive income in GP.
Hackagotchi was meant to revitalize the GP economy by giving people adorable things to covet, make them feel more integrated with the community, and make them feel like they could work toward earning more GP in the future.
When you hackstead, you get a tile of land. You can buy more tiles of land later after your hackstead has earned more XP. You plant things on your tiles of land which yield resources at regular intervals. These resources aren't GP, imagine things like "Hacker Spirit" and "Orange Energy Crystals." Your plants level up after they've been growing for a while. Higher level plants yield more resources and give your hackstead more XP so you can buy more tiles of land. Plants get bonuses to their yield and rate of growth if they're planted on tiles next to plants of the same type, this encourages people to grow only one or two types of plants, meaning that they'll only get one or two types of resources regularly.
In order to craft useful things you need multiple types of resources, so you have to buy and sell from other Hack Club members on the open market. You also aren't given any seeds to start with, so you have to buy some seeds off of the open market as well. There's a small 5% clerical fee when you put something up for sale on the market, and there's also fees on hacksteading. All of this fee money is later "found" by Hackagotchi during events called "Harvests." The innocent Hackagotchi simply give that money to their owners.
You can craft items that give you a chance of spawning different types of Hackagotchi. Hackagotchi give you more GP the happier they are. Hackagotchi can hold items which make them happier. For example, if two Hackagotchi on your hackstead are both holding "Friendship Bracelet" items, you get a small happiness boost. If one Hackagotchi is holding a Rubiks cube, you have a small chance to get a 10% boost to the happiness of your Hackstead when a harvest occurs.