As mentioned on the thread started by RoamingGamer, your game could be the best in the world but without a lot of marketing or a large slice of luck (e.g. someone at apple likes it and puts it on ‘new and noteworthy’ or someone with a lot of twitter followers tweets about it), it’s just one of hundreds of thousands of apps vying for attention.
50,000 won’t pay the bills if it’s a free app, but I’d wager that’s still way above the average, so your game must have something.
However there are a lot of big players in the ‘tower defence’ genre, with huge development and marketing budgets who can make their games much more feature-rich and push their brands harder than an indie developer can. You can’t move for expensive-looking adverts for these sort of games on British TV, and they can also just ‘buy’ 50,000 downloads in a day and brute-force their way to the top of the charts.
The only consolation is that these companies often over-reach and end up losing money even with download figures we can only dream of.