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.
There's definitely low hanging fruit here bitcoin can address. Faster blocks and bigger blocks would easily scale the network and allow it to be used for commerce more easily, and would also solve the cost efficiency problem. For all the promise of lightning network, the implementation hasn't solved any actual issues on a practical level. Its use-case in practice is a very small minority of transactions, so the bulk of transactions in bitcoin remain neither fast nor cheap. On top of that, I'm just not convinced the trade off between decentralization and speed has proven either necessary or worth it. There's not a problem with traditional centralized payment processors to the point where the world
needs a decentralized immutable option. To the extent the world
wants one, there are currently better options than bitcoin due to bitcoin's consensus-based refusal to adapt. In some respects, the consensus system offers benefits, but generally more in what I would call a hypothetical sense as the practical drawbacks at this point continue to heavily outweigh the potential benefits. (Of course, in my extreme minority opinion.)