As tennis is individual sport, tipsters and bettors only have to analyse two players instead of 22+ in football, 10+ in basketball etc. Also the most tennis sites will only represent the ranking and history of any given player, while they neglect other factors like the current form, home ground, traveling, fatigue and probably most importantly health of both players. There are many factors for bookmakers take into account, but also pretty easy to analyse for bettors.
There a lot of markets available for bettors in any major tournament of tennis. All of them are pretty easy to understand and once again, can be often easier predicted than any team sport. Basically, tennis is a sport that is hard to play, but also easy to understand.