Tamerlan died in a shootout after a police officer in Watertown noticed the stolen SUV and called it in. [edit: note there was some tracking of the hijacked SUV due to the owner's cooperation, but that's targeted tracking with cooperation, and not of the brothers themselves]
Tsarnaev was apprehended after an ordinary citizen noticed someone hiding on his boat and called it in.
The massive manhunt, whatever the intelligence apparatus contributed, the shutting down of Boston and suburbs... all accomplished pretty much nothing, other than conditioning people to get used to martial law, and an excuse to point guns at people[1] to cow them into submission.
And what evidence at trial was unlikely to have been available except by parallel construction? The defense strategy was to admit the acts but argue he was brainwashed by his brother and shouldn't receive a death sentence.
[1] https://ajmacdonaldjr.files.wordpress.com/2013/04/martial-la...