When there is a lot of trafic during the onsale of a popular event, the seat map will be disabled. Since there are probably thousands of tickets sold within 1 minute, you simply wouldn't have the time to choose a specific seat. Also, the seat map would have to reload the whole time to give the correct availabilty. After the onsale the seatmap will be added as soon as possible.

Another possibility for not being able to choose your specific seats, is that the promoter of the event has chosen not to offer this option.