I think part of it is the inconsistent airing of H&A. Neighbours follows the same schedule each week, where as H&A in Aus has double/triple episodes sometimes. If they were to catch up, then I imagine 5 would have to switch to Mon-Thurs and try to fill in breaks in the schedule (since H&A produces less episodes).
Neighbours in the UK is actually behind at the moment for the first time in ages. I think about a month? as they decided to only air on Mondays and Fridays when Covid first started. I'm guessing at some point things will catch up again though.
The only thing that bothers me is how it affects fan engagement/discussions. I feel like if everyone was able to watch H&A at the same pace, then maybe more people would tweet or discussions might be bigger.