How to grow a Discord bot
When somebody looks at Discord bot sites like top, they get overwhelmed by the vast selection of bots. Having something special is the key to get a bot on more servers.
What does the user want?
When a user sets up a brand new Discord server, he wants to have it simple. The bot should work out of the box and do as many things as possible. Alternatively, it should do one thing well. When the user is looking for it, he will use this bot.
When the user sees another Mee6 copy, he will probably not use it, except it looks useful and has every feature for free. However, many users go to the Mee6 web page directly because they are used to this bot and want to keep using it. Without asking those users personally, will never get them out of the vicious Mee6 circle.
The Mee6 bot keeps users by having a panel, making it easy to install Mee6 to a new server. So, having a simple UI makes it pleasant for the user.
Why bots fail
The giant bots all have a fully fletched web panel. Having not a panel makes it unpleasant. The user needs something easy and new. If it is free and has all the features a user needs, he will likely keep using it, after getting it recommended.
Bot developers also give up too early. Much advertisement is needed to grow a bot. Pages like top.gg can help out, but only with a high ranking, but a bot will not get a higher ranking without getting many votes. Nevertheless, votes are needed to get a higher ranking. Having many votes at the beginning is essential, to get into the circle. When a bot has a cool feature, which requires that the user has voted, he will likely do that. Doing vote locked commands can help make the bot grow even more after being on several servers.
Conclusion
Users are very likely to add a bot if it is rich in features they need and free. Having a web panel and getting users to vote will help to grow the bot.