Starting with a question, if not for Bitcoin would there be a crypto space? Also, which blockchain is the most secure today? The answer is Bitcoin blockchain. Another question, which blockchain and coin started the main idea backing the crypto space till today? The answer is still Bitcoin, everything falls back to Bitcoin.
Let me explain further, Bitcoin brought about the idea of Peer-to-peer system of transactions which of course takes place in a very secure and decentralized manner; this idea is what many blockchains and platforms are still working with, developing on and improving on, because that's what is needed in the world today.
Yes, there are many other blockchains now but how many can you say is decentralized and trusted. Talking about trust, all these big players, organizations, cooperations coming into this space which coin are most of them buying? Is it not Bitcoin, what stops them from going for others, the simple answer is they know they can have their investments in Bitcoin and be sure of it. Therefore, the more they buy, the more the market cap keeps growing.