I'll give you a list:
He's HG.
He wants to play for the club unlike a number of players in the past (even some that came through the club),
He's shown he can deliver in the PL with a weaker side and showed glimpses in a Blues shirt.
I personally believe he will be a better player than whatever Vlahovic becomes. There are almost two years between them.
Most importantly, HE COSTS NOTHING (ideal for FFP).
We have no idea what will happen over the coming nine months, so I'd rather have Broja as an option to challenge Jackson, who has yet to play a PL minute, might I add, or just for rotating purposes.
There are reports doing the rounds that the club believes the only way to get rid of Lukaku is to sanction this swap. What idiotic way of thinking is that when it isn't even a straight swap?