Thames had one big factor against it - Thatcher, who never forgave them for making "Death on the Rock". Insiders at Thames said the 1991 franchise round arrangement was a vindictive way to make sure Thames would suffer and give Thatcher a victory against them for making that programme in 1988.
Yeah.. this is a bit of a myth along with the Granada/Coronation Street theory.
As has been discussed previously, the main route to winning a franchise was by working out what everyone else was bidding. A way to do this was to behave dynamically as a business, do a lot of networking, offer companies that were potential bidders things like production contracts, a stake in your business, or studio hire. Once you had an idea what you had to bid, you cut your cloth accordingly.
Thames did none of this and was pretty insular as a business, largely due to its indifferent owners.