While this is very opinion based (and likely to get closed for that reason...) I will take a stab at this anyway:
Because he lacks spies and because he does't know how much his brother and sister have changed since he left.
It is said that if you know your enemies and know yourself, you will
not be imperiled in a hundred battles; if you do not know your enemies
but do know yourself, you will win one and lose one; if you do not
know your enemies nor yourself, you will be imperiled in every single
—Sun Tsu, The Art of War (emphasis, mine)
Tyrion does not know his enemies anymore:
- He has not taken into account that Jaime is no longer the proud and "noble" knight that he used to be, who refused to kill Ned Stark in the street, because: "It wasn't clean." Jaime has become cunning and ruthless and no longer lets sentiment impede his better strategic judgement. As Tyrion says himself in S7E05 when speaking with Jaime:
You made me look like a complete fool.
I thought I'd surprise you by hitting Casterly Rock, but you were three steps ahead of me.
Abandoned the family home, completely unsentimental.
Father would have been proud.
- He has no spies in Westeros. This is a serious problem. Neither he, nor Varys (as suspicious as that is...) seem to have any advance knowledge of troop movements or events in Lanister controlled territory, let alone fleet movement! (one would think that with all that water between the Iron Islands and King's Landing, that someone - I'm looking at you Dorne! - would have noticed hundreds of ships sailing merrily along...)
Not knowing his enemies, but knowing himself, according to Sun Tsu, at best gives Tyrion a tie.
Even more speculative speculation about the lack of knowing his enemies follows:
- He does not know anything about the Qyburn factor, since this is a character that appeared and then came to prominence, after the death of his father.
- He may not know just how far down the road of emotionless psychopathic monsterhood his sister has walked since her imprisonment by the High Septon and since the death of her two other children.
One final note, Tyrion is a man of conscience. A man of principle, who seeks to do the least harm in all of his actions. This may be good for a leader in peace time, but in war, he may be getting too soft. If he is going to help Dany win this, he needs to find some of his ruthlessness from the first couple seasons once again...