You need a heck more DLs to see those coming on regular bases. How many images do you have?

What you've seen isn't unusual at all.  Shutterstock sells a lot of subscriptions, and their subscribers tend to download a lot of images.  SS started as a pure subscription site, and they do very well at it both for themselves and their suppliers.  They have been my #1 in terms of revenue for years, only falling to #2 recently as 123RF has started going crazy with both subscription and single downloads.  It helps that SS has no problem accepting lots of "similars", images from the same shoot.  Subscription customers often download a bunch of related images, either to have a choice later or because they haven't filled their download quota and just like them.  Some agencies are less accepting of similars, and their subscription downloads don't do nearly so well.

I have around 200 images. But why is it different with other websites? That's what is killing me. From 30 dowloads on Fotolia more than half are singles but for 80 on SS all of them are subs?

It would appear very difficult to give some definitive answer as to why you get no ODD's at SS.
I have less images than you, yet my totals for April (BME in dl's) were $96 for subs and $60 for ODD's. Some SS'ers seem to get megaSOD's and EL's with amazing regularity but I am always very pleasantly surprised on the rare occasions I get one of the biggies. My port has mucho food and no model released shots.

I find it a bit weird that at most agencies I have a lot of single downloads each month, but never had one on Shutterstock even though I'm selling a lot more pictures there. Let's take the main agencies for the last month.

Fotolia: 11 single downloads and 17 subscriptions
Dreamstime: 5 single downloads and 4 subscriptions
Shutterstock: 80 subscription downloads and...that's all.

Why is this? I was expecting to have a lot of singles for that sales volume...

Fotolia, Dreamstime and iStockphoto all started with a credit-based pricing, customers were coming to them knowing that they are buying image by image (or credits for a few images) while Shutterstock was only offering subscriptions for a long time. This has changed only over the last few years when the credit based sites introduced subscriptions and Shutterstock in turn introduced single image sales and packages. So it is no surprise that traditionally you will find subscriptions are more popular with Shutterstock than with other sites.

However, 80 subs without any other downloads is a bit extreme, though randomly it can happen. My stats this month is 167 subs sales, 11 on demand downloads and 2 single images. However, the single and on demand are making about a third of the royalties. Since I started a few months ago, I have a total of $250 coming from subs and $248 coming from other downloads, almost an even split.

For me so far this year I have been getting an OD about every 11-20 DLs, depending on the month.  This month it is the slowest at one OD per every 25 DLs.  So 80 DLs with no ODs is very unusual in my experience.  However, it is probably just statistical variation in a small port - just keep uploading and the ODs will come.