Why vestibules on locomotive-hauled cars but not EMUs?

I assume that the reason is due to the placement of the motors, but:

Why do locomotive-hauled passenger cars have vestibules at one or both ends and exterior doors usually (but not always) in the vestibules, but EMUs (at least on the Long Island Rail Road and Metro-North) don't have vestibules but have doors that enter directly into the passenger compartment?

Thanks.

low level steps, they can really not be put between the trucks as it impacts the strength of side sills.

If you're talking about end-doors on EMU's, the Arrow III's on NJT and all but the newest Silverliners on SEPTA have end-doors and they are MU's.

True. I had forgotten about the older EMUs. Thank you both.

I'd guess, putting two and two together: doors towards the middle of the car allow for better entry/exit, and so cars that serve low-level stations would ideally have them that way but can't due to car structure issues?

I think there has recently been some issue with the FRA and end-door vestibules (something to do with crumple zones on a car during a collision). For example, the recent Silverliner V's all have quarter-point doors with traps in the middle of the cars.

Judging by the Silverliner V order, I would assume that going forward that most MU's will all have quarter-point doors (doors in the middle of the cars) and end-door vestibules will become a thing of the past.