To entice you to buy the more expensive model. Plain and simple..
How enticing is just that one, specific feature? How many people outside of this message thread are even aware of it?
I would totally believe that Apple would hold back a feature for that purpose in certain circumstances, but it doesn’t seem to make sense here. Why not hold it back throughout the UI?
Here’s my attempt at an alternate explanation: Apple has determined that long-press on lockscreen is somehow more prone to accidental keypress (“butt dial”) than 3D Touch, and they held it back to avoid those actions being triggered without the user’s intention. This wouldn’t apply to iPad, since it’s sitting on the table.