I work at an online game company and 99.9% of the games on mobile (including tablets) suck. They are just bad games. That said, mobile developers are limited because of the fact that there is no option to have a physical controller for your average player... you have to make games (for the average player) that users must use their finger/tablet pen for. It sounds like it wouldn't be that big of a deal, but it is. I have an iPad, but to be honest...I only use it to watch Netflix and YouTube. For games, I use my console systems or do PC games.
So even if you do have an AMAZING game, you are limited to the fact that the user must user their finger/pen and not a controller.
Then after that, you have the fact that nearly anyone and their mother can upload games to the android app store (iOs is more strict). Most games you find are rip-offs, fads, and bad replicas of games that have been made already.
You CAN find decent games. Search for games you know you already like or, you mentioned you like tower defense, maybe go online and google "best tower defense games for tablets" etc.
Since I play most of my games on PC or console, I don't have much advice of which particular games, but I do know the industry and it is SATURATED with bad games. It takes much less money and man-power to make a mobile/tablet game than it does to make a console game. That is one of the reasons why there are SO MANY of them and why most of them suck.