As for why nobody else bid - there was probably nobody better placed than Central themselves to serve the region; the business was sound (they had no sprawling assets like Granada, nor any risky investments like TVS) and the changes insisted by the 1990 act didn't represent a threat to them (Central were already doing quite a lot of independent commissions - particularly with the Childrens' output). The only weak spot was Oxford, which they had moved to protect in 1989. In short, there was no question of their ability to win, nor their ability to finance themselves - so it would have been suicide to bid against them.
The other week I bought Independent Television In Britain 1981-92 from Oxfam in Notting Hill (they have a different kind of Oxfam in Notting Hill) which obviously goes into the franchise round in great detail. And Central were unapposed for the reasons you suggest, they were working with a lot of independent companies. The one consortium that was pondering a bid for the Midlands was the one headed up Mentorn, who were eyeing up either LWT or Central, but Mentorn were making programmes for Central. Central wrote to all their existing indies to ask if they were happy to be included in their bid, and when Mentorn didn't reply they got in touch, arranged a meeting and promptly offered them a new contract, at which point Mentorn decided to bid against LWT instead.
They also commissioned a lot of market research because they neighboured so many regions and they felt there was a potential for so many people on the edges to say Central were crap at serving their area and a lot of the neighbouring regions to make use of that to bid against them. In the end it turned out everyone was pretty happy with how things were going apart from in the South of the region, which is why they launched the South opt-out off their own backs, knowing full well that any opposition would have to match that. OK, so it was based on the cash bid but that was surely one of the scenarios where exceptional circumstances would have come into play. So nobody fancied the expense of running three sub-regions, and most people in telly in the Midlands were already happily working with Central, so nobody went up against them.
It was a bit different when TVS launched the separate show for the South East - that wasn't something they offered, it was stipulated they would have to provide it as a condition of the franchise. Southern would have had to have done it had they won it.
As for STV, again most of the big Scottish indies were already working with STV, and though it covers a nation the Scottish media industry is quite small - you only have to see how many people worked for both the BBC and STV - so if there had been someone bidding against them they'd have known about it. The managing director also went to the board and their non-executive directors, who were all merchant bankers and solicitors and so on, and had been deliberately picked to be the best in their field, and so it was assumed they would easily get wind of any other bids.
As for Thames, the book points out that the ITC were seriously considering letting Thames through on exceptional circumstances, because the Thames plans were considered exceptional, but one of the issues was that they weren't supposed to judge on their existing programme line-up because that would mean the incumbent would always win. Thames could point to the programmes they had already made, but Carlton couldn't, so it was an unfair contest. They decided that Carlton had some very talented people working for them, and had done deals with independent companies with very good track records and with a reputation for quality, so while the proposals weren't quite as good as Thames, they were certainly good enough.
In the book too David Mellor says that nobody ever really worked out what "exceptional circumstances" meant and it was up to the ITC to use their skill and judgement as to what it might entail. He suggests that if Thames had made a really low bid but instead pledged to spend many millions more on programmes, that might have been considered the "exceptional circumstances". But David Elstein says they thought about doing that, but thought they should make a competitive bid so it looked like they were taking it seriously and didn't come across as arrogant, relying on their name and heritage.