First of all it's probably hard due to big amount of data. Secondly (the main reason), they don't think that development is needed because Bitcoin is the most popular crypto anyway. That's why Vitalik Buterin is constantly evolving his Ethereum that is currently way better than Bitcoin.
I agree, changes might affect the way Bitcoin is seen by the people. Change too much and you are doomed, change too little and people will think you don't care enough about it.