Because nothing happened in 2018 and the "Blue Wave! Blue Wave!" narrative is, was, and always will be across the board nonsense.
The opposition party almost always makes gains in the midterm. It's not a symbol of anything. It's a slight re-balancing of the scales following the more popular and passionate Presidential election at best, a political dead cat bounce at worst.
Two years after Barack Obama was elected for his first term the Republicans gained 6 Senate seats, 63 House seats, 6 Gubernatorial Seats, and flipped the balance of power in 20 state legislature houses.
Two years after Barack Obama was elected for his second term the Republicans gained 9 Senate seats, 12 seats, and 2 Gubernatorial Seats.
Two years after Donald Trump was elected for his first term the Democrats lost 2 Senate seats, gained 41 House seats, 7 Gubernatorial Seats.