I suspect they don't have that information easily available and have no desire to create more work for themselves.
Think the season ticket points get added manually hence every season we get the 'have your season ticket points been added yet' and it seemingly taking weeks for everyone to get them.
You could pull the data to work out who had points and who to take off as season ticket holders but then how do you make a sale only available to them?
The ticketing system has your season ticket purchase on the history iirc. I bought a season ticket to donate (I live abroad) a couple of times in the last few years and it's on my history... they did remove the points though

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You're right though, any time a points threshold is introduced there is a bit of a scramble because people realise they're missing things and that must generate load on the ticket office - but we have had multiple "big games" in the last few years where points thresholds were a blocker - and therefore the userbase is probably in pretty good shape.
I know it's work but also the club have been improving operations a lot - this sale was about as careless as it could be - a small number of tickets in high demand released without any prior warning or communication, during working hours with no prioritisation in place.
I was the beneficiary of that, but feel for others who missed out.