Regardless of why, the inflexibility of bitcoin is why it's still a slow and clunky crypto and why it hasn't been able to incorporate improvements that other crypto's have introduced.
Bitcoin changes slowly, yes. But it's not going to change faster if it's threatening any of the main pillars that I mentioned above. Bitcoin changed to incorporate SegWit, we now have the Lightning Network, and soon we will have Taproot and Schnorr signatures.
When you talk about the improvements in altcoins, those changes can make them faster and cheaper, but to they make the networks more secure? Many altcoins have suffered 51% attacks and many others could be subjected to them with very little cost. Bitcoin doesn't do rollbacks, altcoins have. Bitcoin isn't centralized, many alts are + they are premined.