I should imagine that the simple reason is that BMW will sell considerably more iX1’s than i4’s in the marketplace, so they don’t need to be as generous. The growth in demand for compact SUVs has been phenomenal, whereas the market for coupe style saloons is far more niche.
I should also imagine that BMW figured that the iX1 would be a far more practical solution for many with mobility challenges than the i4, so they could get away with charging comparatively more. Motability customers will be far more interested in practicality than what the retail selling price is, whereas private customers will be extremely focussed on price where, again, the iX1 wins and is therefore likely to be far easier to sell.
However, the arrival of the iX1 on the scheme has met with virtually no interest amongst forum members, so hopefully demand will be low and BMW will therefore consider adding a pack or (more likely) lower the AP – or both!