Because a suit is (sadly) used only as a corporate uniform designed to invoke power and appeal to authority. It isn't intended to be a useful and functional item of clothing. The suit evolved and was to a large extent designed. It was intended to be a leveler and to democratise mens clothing outfits. It sad that the most democratic of outfits - the suit - has come to be regarded in some places as a signifier of elitism. 99% of suits are worn in air-conditioned offices which is why you can wear all season fabrics all year round. It should be linen/cotton/open-weave wool in summer and flannels/corduroy in winter, with 'all-season wool (i.e. generic super 100-120's) used in autumn and spring. But people don't think about their suits in that way. Or clothes full stop for that matter. I'm sure I'm not alone but I have had jobs where I emerge from air con house to walk maybe 5 metres to an air con car, then drive some way to a cool underground car park and then enter an air con building to work all day . Then at evening the performance is repeated, from Air con office down to basement, to air con car then from car to air con house. The only time one needs to adjust is for a short walk outside or if one is on public transport. And increasingly public transport is air con too. The suit, and most other classic mens outfits emerged from UK and for many years of its evolution London had inadequate heating and insulation hence heavy materials and 3 piece suits in office and negligible summers - compared to our summers. Mens clothes haven't evolved much for our climate, although one could look to the italian influence of lighter materials, sport coats instead of suits and lighter shoes as perhaps evidence that there has been some evolution in Oz. Part of the problem is that sunnier climes encourage less clothes generally and this coupled with a small - ish market means there isn't a lot of push or pull for an evolution of formal (business) wear in Oz.