Manchester City supporters have pointed out the price of tickets, ranging from £30 to £150, the cost of travel down to London, buying food and drink and a Sunday kick-off at 16:30 BST as the major factors for the number of empty seats.
Kevin Parker, general secretary of City's official supporters' club, told BBC Sport: "It is an expensive day out and when you have done it 25 or 30 times, there are decisions you have to take in terms of the cost.
"Our record in the FA Cup under Guardiola has been unbelievable, but fans - without disrespect to Nottingham Forest - are hoping to be reasonably confident we will play in the final. Those games are only three weeks apart. People do have to make a financial choice."
Those who were in attendance joyously celebrated victory with their 'Poznan' dance, while Guardiola took the adulation of the travelling supporters.
"You could see the passion within the supporters after beating Nottingham Forest, but 'Wembley fatigue' is a good way of describing how we feel about the day," says Parker.
"It is a long day for travel, the additional expense and having work or school the next day. It is inconvenient to play at 4:30pm on a Sunday."
Some City fans are planning further protests during the Premier League game against Wolves at Etihad Stadium on Friday over what they say is the club's refusal to increase the number of season-ticket holders.
But Parker said the non-sale of tickets for the semi-final was "not a fan protest" and the empty seats had "nothing to do with that".
The official supporters' group has 400 branches worldwide and Parker said it already had ticket applications from places such as Thailand, Iraq, Australia and the United States for the final.