That will be partly because the turnaround times at Leeds have increased from 30 to 80 minutes, therefore fewer round trips are possible in a day and the diagrams have become less efficient.
So now occupying a platform for 80 mins? That's madness in such a busy station.
The LNER rolling stock is also very expensive to operate and LNER (in contrast to Avanti and Greater Anglia) have an operational deficit so this is not great either for rail passengers or taxpayers. It is clear that LNER do not have enough rolling stock to run their December 2025 timetable with long enough trains in all cases. As well as the IC225s they will also be relying on running many 5 coach trains.
I wonder if it would be better in future timetables for the two LNER trains each way each hour between London Kings Cross to Leeds to run to Leeds only and let another train operator run all train services connecting Leeds with Bradford and Harrogate. Then perhaps the LNER turnarounds at Leeds could all be 50 minutes which would be a more efficient use of the very expensive rolling stock.
I suppose those who want the direct LNER train services Bradford and Harrogate to London Kings Cross would not like this but providing those direct train services is looking like a very expensive and inefficient use of LNER's limited rolling stock.
I can understand the decision not to risk a 20 minute turnaround as it is essential that the December 2025 East Coast Mainline timetable works reliably and does not fall apart.