Why don't Australian magpies in Tasmania swoop at people?
I cannot give you a proved explanation of why magpies don't swoop in Tasmania. Nevertheless, my goal here is corroborating your impression with some data, and indicating a possible cause.

Aussie Magpies and swooping

Australian magpies are not the same species of European magpies (Pica pica). Actually, they are not even corvids. They belong to the Family Artamidae, and their scientific name is Gymnorhina tibicen (synonym: Cracticus tibicen). Here is a male magpie (you can tell he's a male by the white colour in his back):



Australian magpie in Victoria

Magpies are territorial birds and in the breeding season (which is right now, as I write this, in September) a small percentage of the males become quite aggressive towards people approaching the nest. 

According to the Wikipedia link above:


  These magpies may engage in an escalating series of behaviours to drive off intruders. Least threatening are alarm calls and distant swoops, where birds fly within several metres from behind and perch nearby. Next in intensity are close swoops, where a magpie will swoop in from behind or the side and audibly "snap" their beaks or even peck or bite at the face, neck, ears or eyes. More rarely, a bird may dive-bomb and strike the intruder's (usually a cyclist's) head with its chest. 


Cyclists are well known targets:



As a personal and anecdotal note, I have to say that I bike during the breeding season and I was never attacked by a magpie, nor I've ever seen a magpie swooping.  

No swooping in Tasmania

Your claim sounded quite incorrect to me when I first read your question. After all, the magpies in Tassie (short for Tasmania) and the magpies in the mainland belong to the same species (G. tibicen). 

However, after some research, your claim seems to be correct: there is very few, if any, report of magpies swooping in Tassie.

For instance, this is a chart from the Magpie Alert website:



As you can see, there is no registered attacks in Tassie (TAS) from 2013 to 2017.

And here is a map of the attacks (the online version is here):



Most of the attacks happen in Victoria and New South Wales. However, besides Tasmania, you can see both in the chart and in the map that there is no attack registered in Northern Territory (NT) and New Zealand (NZ) as well.

That data doesn't prove that there is no swooping in Tasmania, but it does show that, somehow, Tasmanian magpies behave differently. Maybe they are more shy, or maybe the males over there perform other behaviours to drive off intruders before swooping. It's also possible that the explanation is completely different: maybe people in Tasmania don't like to go online and complain about magpies. 

A possible cause

The Tasmanian magpie (G. tibicen hypoleuca) is the smallest magpies of all in Australia. Besides that, it is the magpie that forms the largest groups (Kaplan, 2004), with as many as 15 individuals.  



Tasmanian magpie, also a male

Coincidentally, magpies form smallest groups exactly in the areas of higher incidence of swooping, like Victoria and New South Wales. The possible correlation among those variables (swooping rate, body size and group size) is something that is still to be investigated. 

Conclusion

However, regardless the actual explanation, the fact is that the number of registered attacks in Tassie is, indeed, smaller than in the mainland. After doing a lot of research, the explanation for this remains unknown to me. Let's wait for more answers.

PS: as I said in my comment, magpies are not that aggressive: they are very nice birds and very intelligent ones as well. So, one way to avoid attacks from the magpies in your area is becoming friends with them: magpies have a good memory and they can recognise a face after years. So, if you become friends with your local magpies, they probably won't attack you if you get closer to the nest. By the way, the best way to making new friends is with food (that works with humans too).



Source: Kaplan, G. (2004). Australian Magpie: Biology and Behaviour of an Unusual Songbird (Australian natural history series). CSIRO Publishing.