There are at least two reasons for this.
First, higher income people will likely be more able to get time off to go and vote. They will tend to have jobs that are more flexible, thus allowing them to be able to go to a polling place. Thus, it is easier for them to vote even though their time is more valuable.
Second and perhaps more importantly, higher income people get more benefits from voting. People benefit more from voting if they feel the system works for them. If they feel that way, they are more likely to gain satisfaction from participating in that system. Higher income people are more likely to feel this way because they have succeeded in life and the system has generally worked for them. Therefore, the benefits they gain from voting are more likely to outweigh the costs that they incur.