The Package holiday business is very volatile and TC's competitors were much better at predicting / reacting to changing holiday trends. TUI and jet 2 spend a lot of time analysing customer behaviour and trends such as "what food are people buying from Tesco?" in order to predict peoples aspirations ahead of booking a holiday. So if people are buying up lots of Mexican food from Sainsburys, they can anticipate increased demand for Mexican holiday destinations. If people are buying Greek food then there may be "spike" in Greek holidays.
I know it sounds ridiculous to make decisions based on Tesco clubcard data, but they actively want to hear about what OTHER destinations people are looking at to get ahead of the game. Jet 2 are particularly strong on their IT and Analytics. TC was also trying to maintain a large physical network of shops which undoubtedly added bigger costs and potentially hindered their ability to adapt / change systems to better capture customer behaviour.
These new or emerging destinations may have limited accommodation resources and if you can get in and secure the majority of rooms at a hotel in a sought after location early, then your competitors have less slice of the action. Sometimes a destination can be a flash in a pan / flavour of the month type thing and they will just look to utilise existing accommodation. If they think a destination has longevity they may get into partnership / borrow money to build new accommodation but that can be higher risk as TC found out.
Thomas cook had invested VERY heavily in Turkey and events in 2016 in the country meant Turkey plummeted in popularity overnight. Jet 2 had a huge presence in Turkey too but they seem to have been able to extricate themselves from this quicker / cheaper than TC did.
I also think that TC took a wrong turn around this time due to cost cutting. we had used TC for several holidays but all of a sudden the seats got smaller / less cabin room. The accommodation on one holiday we took was absolutely filthy (TC had just sacked the Cleaning team that had created previously excellent reviews) and they didn't respond to my complaints about cockroaches in our room.
The resort staff were just really bad as well - looking totally disengaged - something I'd never seen before on a TC holiday. We started using Jet 2 and I wasn't surprised to hear when TC went under a year or so later. Things were clearly in a big decline.