There are a number of possible reasons for that. In the post-war era there was a desire to rationalise everything. It was a reaction against everything from medieval street layouts to the slightly florid style of Victorian architecture that became so predominant at the end of the 19th century. Planners and architects wanted to do away with this higgledy-piggledy mishmash of periods and create something that was clean and functional. Modernist architects since the 1930s were united by a love of flat roofs, glass walls and steel or concrete frames, and so that language of modern architecture helped create a new standardised look for buildings, just as the neoclassical or neogothic styles had the century before.
At the same time, post-war developers wanted to maximise the return they could get on the plots of land they'd acquired. So they started employing large architecture practices who had become particularly good at exploiting loopholes in the planning system. Dodging these rules allowed them to maximise the amount of floorspace you can get on a particular plot.
"The most famous of these architects was Richard Seifert. He had a massive company that designed office blocks all over the country. So whether you’re in Glasgow, Birmingham or Leicester you see towers designed by his company."
The most famous of these architects was Richard Seifert, the designer of Centre Point and NatWest Tower. He had a massive company that designed office blocks all over the country. So whether you’re in Glasgow, Birmingham or Leicester you see towers designed by his company built using standardised components that help create a familiarity or regularity.
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And it wasn't just office blocks, an even more ruthlessly commercial thing happened with companies building shopping centres. Arndale would employ specialist architects who knew how to design these vast consumerist machines. Their primary concern is functionality. When you had companies wading in who didn't know about this stuff, you ended up with buildings like The Bull Ring in Birmingham. In fact, the one thing you can say in the defence of the Bull Ring was that it was clearly designed by people who had not designed shopping centres before, which is why it was such an awkward building, and why they had trouble letting all of the shops.