What is the main reason of this closure? Is it the park not identified pests in the structure or just not replaced worn out sections of the support/track over the years? As I've read the topic and seen few things put forward, unless I did not see it?
It's probably a combination of mechanical and biological factors (one could rule out most of the others - thermal, chemical, or electromagnetical factors shouldn't be prominent enough to degrate the structure significantly). The mechanical part is pretty self-explanatory, but the biological component could be fungi, worms, or even woodpeckers.
Either, the engineers haven't done a good enough job assessing what kind of degradation agents were present in the coaster's area (for instance, failed to see if the nearby trees housed beetle colonies), or they have miscalculated the impact of the agents present. For instance, they could have been assured that the wood was treated against fungi or worms, and told that they wouldn't need to inspect for that or treat the wood more often than, say, once every three years, when once a year would actually be required (just making these numbers up, by the way). Actually, I wouldn't rule out action against Intamin or whoever delivered the wooden structure, if it turns out that the park received insufficient maintenance information when the coaster was built.
Colossos is built using laminated timber (not sure if it's just the track or the whole structure) in 2001, back when laminated timber was a fairly new invention. I believe its usage in the construction industry only took off in the 1990s, and mostly for indoor use. It could be that the manufacturers had insufficient experience about the required treatment for outdoor environments, or the industry standards were too lax, resulting in the coaster failing to achieve its projected lifetime. It would certainly not be the first, or the last, time a construction product was misused soon after hitting the market. Scandals involving new products that fail to perform are a pretty much constant phenomenon in the construction industry.
Or it could simply be neglect on Merlin's part. Insufficient training of maintenance personnel, insufficient quality of replacement materials, insufficient yearly treatment of the wood... there are lots of opportunities for degradation of wood, since it is a biological material with no means to protect itself. Lots of things in nature consider wood food, so long-time use of wood in an outdoor environment requires a lot of steps constantly taken to keep the gluttons away.