indeed the most important thing is because bitcoin is the oldest coin, therefore it seems that it has got a brand, so that in its development it has gained great trust from investors. and that trust is what makes investors seem to unite their thoughts to invest in bitcoin, so that other altcoins seem to follow the movement of bitcoin because of the influence of the thoughts of each investor.
Bitcoin definitely has the first-mover advantage, essentially it allowed bitcoin to form a strong decentralized foundation on which the other very important characteristics can be built. If we think about it for a moment, without decentralization, there wouldn't exist such important value propositions of bitcoin as a limited supply, predictable issuance that algorithmically dictates bitcoin's monetary policy, irreversibility of transactions, and the highest possible uptime. In contrast to bitcoin, the vast majority of altcoins, if not all, haven't achieved true decentralization because (1) their founders are always present and have enormous influence on the project they created, (2) users that join such projects are usually short-term oriented seeking only for profits. Those, who don't care about the future of a project or don't even believe a project has a future, cannot provide for decentralization or can't sustain it for long.