would wager its shit financial management, they gave their record signing a 3 year contract ffs, and pretty certain a lot of their players who came down didn't have relegation wage cuts in them, hence the likes of ngog staying there for ages
They were heavily indebted when they were in the PL too, as I recall David Conn and the Swiss Ramble in an excellent piece wrote about them quite a bit. The
Swiss Ramble noted back in 2010, that Bolton lost £35m during that year and their net debt was £93m, due to low turnover and a high wage bill. In 2006, their debt was at £29m, but it tripled, rising from £64m to £93m between 2009 and 2010 alone. As Swiss Ramble stated in the 2010 article, Bolton were paying around £4.5m a year in interest payments on an annual turnover of only £60m. Originally, most of their debt when Eddie Davies bought the team, was accumulated from building the Reebok Stadium and the adjoining hotel complex. The later debt largely grew from wage increases and player amortization. By 2008/09, 72% of Bolton's income was TV based. Only Wigan were higher (81%)
Their 'model' if you will was a bit akin to Fulham's for a while: try to squeeze the last bit of brilliance from aging players with quality (Okocha, Djorkaeff), with hardworking castoffs (Davies, Stelios etc.). The problem they ran into, at a more extreme sense than Fulham, was that since they hardly developed any young players from their Academy of any worth to the first team (Nolan being an exception), they were constantly reliant on paying high wages for players mostly on frees or cut-price deals, who little to no resale value. Ironically, the Swiss Ramble noted back in 2010, that the two clubs outside of the Top 4 who had the biggest debt were Fulham and Bolton.
The most amount of money they ever recouped for a player was Anelka (£15m) followed by Cahill for £7m, with the latter leaving at a cut-price given he was going to be able to leave on a free at the end of the season. But even those two players still rank as the second and third highest fees that Bolton ever paid (£8m for Anelka and £5m for Cahill) for a player. The third biggest fee Bolton have ever received is still Jason McAteer in 1995 (£4.5m)!
Their record signing (Elmander at £8.2m) was 27 when they signed him. N'Gog is tied for fifth in the highest transfer fees ever at £4m. Given that Allardyce had plenty of control over every aspect, it was always not going to take much for that ship to rock. Once Sammy Lee came in and tried to play possession football, they struggled and having the likes of Owen Coyle and Gary Megson in charge wasn't going to help, as at least Allardyce was progressive in the statistical and sports science end that gave Bolton an advantage in pulling above their weight.