Thinking about it, is the reason it's a slot rather than a simple circle is for traffic control reasons? You can put green lights on one side of a slot, red on the other (and reverse them on the inner side of the slot) to provide a 'correct' entrance and exit in moving three-dimensional space. It's easy and intuitive for pilots to see and understand, and for traffic controllers to call out instructions. If it was a circle, then it's much harder to give people a relative position. Even sticking green and red lights on opposite sides of the circle is a bit fuzzy. Because a slot has defined 'sides'. Even if you wanted to take advantage of the structural advantages of an elliptical slot (which wouldn't be as good as a circle, but would have some of the advantages of a slot), that would throw up other issues - a slot is the same 'height' all along its length, so a ship knows it can be a 'left' and 'right' of centre without any problems... it only has to worry about clearance from the edges - with an ellipse, it narrows towards the edges so pilots in big or tall ships would naturally drift towards the centre out of fear, defeating the traffic control idea. The entrance really has to be the same height all along its length for consistency and so there's no fuzzy vagueness about whether a ship will fit through it 'about a 1/3 of the way along its length, but not any further'.
As an aside, I imagine there's probably specific jargon to talk about the 'sides' of a slot, since left and right wouldn't work from a rotating slot that can be entered either way up from both directions. The most obvious way to distinguish the sides of the slot would be 'Outbound' and 'Inbound', tying the colour to specify the side to its function, regardless of whether you're inside or outside the station.
I also now can't get the ridiculous image of a bunch of harassed exhausted traffic controllers in the spike watching the entrance:
'Adder departing outbound from pad 37... clear of slot... Sidewinder entering inbound to pad 9... clear of slot... oh god why stop there?... Type 6 entering inbound to pad 42... clear of slot... what the... Cobra entering OUTBOUND to pad 7... clear of slot, near collision with Sidewinder... some of these commanders where'd they learn to f - OH GOD TYPE 9 TYPE 9 TYPE 9'