From the official Warner Bros press release on The Batman, literal first sentence:
This Bruce Wayne is a billionaire, but even a billionaire that's trying to do this stuff in secret all by himself would probably not come out with something super advanced in his second year on the job. This is a two men operation in which he has to maintain his identity a secret and make things that are practical for him. This suit, this batmobile, follow suit.
As great as the tumbler and many other Batmobiles were, it was also more than obvious they could only have been made by a rich person. Not to mention; you see the Keatonmobile driving around, you see the tumbler riding around, you immediately know something's up, police would immediately start chasing you, criminals would instantly recognize you.
This is a detective Batman, and as a detective, sometimes he's gonna have to enter crowded streets to investigates, sometimes he's gonna have to tail people silently, he's gonna have to patrol streets, spend hours and hours parked just looking at a single place without drawing too much attention. This vehicle is literally perfect for that. If you go out there to the streets at night right now and imagine this thing riding around compared to imagining the tumbler or other vehicles, this makes perfect sense for a grounded Batman.