It's hard to convey why you lost because it's often the result of a lot of mistakes that accumulate over time, such as the lack of building any income.

That said a lot of tower defenses have a sorta clear reason why you lost, such as a boss or a type of enemy you didn't deal with properly. In BTD6, if you lost from a big zeppelin then you don't have much single target damage. If you lost from camo bloons, then you lack camo detection, etc. It's usually fairly obvious why you lost, and if you have multiple lanes then maybe you didn't care for a specific one.

Can you give an example of a game that does this? I’ve never seen anything like what you describe.

BTD6 would never :'3

I think there's 2 good failure states, one is you learn something from why you failed so you want to try again to update your strategy and see if it works. The other is you carry over advancements or abilities from previous attempts so even if you try to brute force it it eventually works. Or I think most games have some mix of the two. For the first style having a recap at the end that shows a summary of how you did and also what you did so you can see if it improves with different actions seems less frustrating, like a damage parser you can only see at the end when you lose or win, or just a count of abilities used or structures built or whatever.

It's not quite traditional tower defense, but Bad North is great, and scratches the tower defense itch for me.

It shows the map and enemies you will be up against each level, and if you lose it's always clear why. (sometimes it's because you got greedy or were stretched thin and didn't send enough units, or because you didn't choose to send the right ones, and often it's because you made a tactical error while fighting. But it never feels like the game is being randomly unfair)