Mining centralization is a big Bitcoin problem, and the development of ASICs is one of the main causes of this, but I haven't found a lot of discussion about changing the mining algorithm to a more ASIC-resistant one. Is this because there is some kind of contract between the big miners and the Bitcoin developers, to not change it ever? Or there is some work in progress?
Short answer: no.
Remember, that it's not really the developers who decide. A mining algorithm change would mean a hard fork, which is an incredibly difficult task to pull off. The developers develop on behalf of the community, which is comprised of miners, exchanges, merchants, customers, and other types of developers. Each of these groups needs to take the needs of the other groups into consideration as well as their own. In general, the potential problems with a hard fork have outweighed the benefits of changing the mining algorithm in the eyes of each of these groups.
Also remember that the concept of sidechains is on the roadmap as a possible scaling option. Any of these sidechains could use whatever mining algorithm they choose, which would help to solve mining centralization in their own way.