The main issue is that Merlin have built a rod for their own back.
For years, they've pushed Merlin Annual Passes, as well as park-specific Season and Annual passes, down everyone's throats for years. They have a high on the gate price, and make the passes look as lucrative as possible. I've no idea what the numbers would be like, but it wouldn't be a surprise if a huge percentage of visitors are Annual passholders.
But they're backed into a corner at the moment. With capacity limited at the parks, they don't want to risk having huge proportions of people turn up, not spend money to get in, and spend minimal money when through the gates (passholders traditionally spend less, especially if it's their second or more visit). Because all that means is they're making less, or perhaps even losing, money. And at a time when they were closed for 4 months, that's not good.
If they hadn't so aggressively pushed passes over the last few years, and had less people out there with passes, it wouldn't be a problem. But as it stands, it is.
The problem is further highlighted now because passholders don't have a pre book fee. At first, you had to pay £1 to pre book a ticket (it was effectively a way to deter people from booking and not showing up), but that's been dropped as of a couple of weeks ago. Not that the £1 fee stopped people no showing anyways..
The trouble is, what can they do? They've got this problem of huge numbers of passholders who will likely want to book, and not wanting to give away too many spaces to them. They can, as they are doing, release more close to the date if they haven't sold out, but that doesn't solve the problem. And, as much as I hate the pre booking and planning months in advance, I understand the want to limit numbers.
In my opinion, Merlin should be extending passes for longer. I believe they system they had is that you get 5 extra months added to the pass, not including closed season. So if your pass ran out in April, it was extended to September. If it runs out in October, they extended it to August (ignore close season, then add 5 months from start of season in March). (?)
The trouble is, everyone has been affected by the booking issues since parks reopened, and even if you get that extension, you might not have been able to visit parks. So you get frustrated and lose faith. I'd have thought giving everyone an extension until mid-2021 would have been a solid idea, as you have more of a chance of things being closer to a sense of normality and help keep customers happy. Sure, you don't make as many sales of new passes / renewals, but you make up for that in people still visiting the parks, spending money, and get the big hit in 2021.