Because I already explained why?
Batman:
-Lives in Gotham
-Muscular
-Obsessed with stopping crime
-Has advanced tech, therefore rich
-Has a lot of skill, therefore is rich enough to educate himself
Light simply needs to build a profile of Batman
Bruce Wayne:
-Lives in Gotham
-Muscular
-Lost his parents due to crime (possible motivation)
-Is rich and owns a company that sells advanced tech
And Bruce fits the bill
By just....investigating
Again explain how these help. Simply saying "better tech" or "smarter" is not gonna cut it
Batman has actively proven he isn't Bruce dozens of times and to the media.
He's crafted an alibi, I mean obviously, you came up with that yourself in minutes, most rogues have too so it's something he's made blatant isn't true.
There's also the issue that most people don't actually know Batman uses tech, many, including government agencies think he's a meta human like a vampire or demon.
Anyway Ryuk straight up screws Light over given Batman could sense, if not see him outright.
Batman is going to try to handle the case alone to start so the Japanese Police won't even know Batman is there yet, he'd quickly deduce the same stuff L did on his plane ride over, pull up every single record that fits, bring it down to a few dozen suspects and go stalk them out first quickly, probably hack into basic stuff like street cams too and would likely already suspect a supernatural killer given:
Batman: Gotham County Line (2005)
Batman: Gothic / Legends of the Dark Knight #6-10 (1990)
Batman/Deadman: Death and Glory (1996)
Batman: Dark Knight, Dark City / Batman #452-454 (1990)
Batman: Demon (1996)
Batman #540-541, "The Spectre of Vengeance" (1997)
That's to say, Batman basically solved a Kira-type case multiple times and he's only gotten way better since.