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I imagine it's because the M252 is manufactured in China. The M254 is getting on a bit and is I think made in Europe. The M139 is an AMG motor that's not going to be made in enough volume (though I do think a detuned factory assembled version would be a good idea).
They'd need time to spin up a factory in the US to produce enough motors to meet demand, and crucially, avoid Tariffs.
All of this is about tariffs. If they weren't a thing, I don't think Mercedes would bother. And the B48 is a fine motor, at any rate.
The M139 isn't Euro7 compliant anyways. As funny [and embarrassing for some] as this is, the B48 is the best I4 around imo so it will be an improvement.
The Mercedes engine portfolio is newer than BMW engines. It didn't make sense to me.
If the economics don't work it doesn't matter how new MB's engines are. It makes sense to me that this only for I4s. Mercedes has been mismanaged since the pandemic and this is the price that must be paid to get out of the mess.