1) Like Cayman vs. 911, they were trying not to cannibalize the more expensive model – trimming power leads to an outcry, making the cars feel distinct from the driver’s seat via spring/shock/bar/effective steering ratio less so.
2) The 1 series was hammered by the press for its hobbyhorse ride (run flats+short wheelbase) in its original hatchback form, so this midcycle refresh required they stress ride comfort (same happened with X3 and Mini) which in knee jerk engineering terms means lower suspension natural frequency.
3) They were trying to go for more of the vintage BMW feel (long travel, real world compliance, roll to signify how close the limit is and make sure it’s exceeded progressively) – more along the lines of an E30 or a current Xi than an i or sport package car (which feel ‘slammed’ out of the box and mistake grip for handling). There’s a sense that since they couldn’t make it lightweight like a 2002 they wanted a bit of the feeling that died with the last of the trailing arm cars… Also note the narrower tires! More self aligning torque, more progressive breakaway… just what all BMWs need these days!
4) The 1 series has a lot of baked in understeer to protect those not used to RWD - I’ve read many a test that mention cars like the Mini or even the Golf are more neutral (I suppose they can be because torque reversal at the limit can’t upset the chassis so much).
That’s my theory – and I’m sticking to it.