I was thinking of this not so long ago. My take is the combination of the factors you named. Let me explain.
When people first come to crypto - they start with bitcoin. When people get familiar with altoins - they face the need to transfer bitcoin to exchange to trade alts.
Before November 2017 majority of people didn't pay attention to the crypto they used for transporting value between exchanges (or wallets). BTC was the default choice due to:

• it's (holy) Bitcoin, not a shitcoin!
• all exchanges have always more BTC based pairs then to any other crypto
• all crypto trading tutorials advice to count profits in BTC
• etc

It was all good until people faced extremely high fees in November-December 2017. Masses tried to use other means of value transferring, like ETH, LTC, STEEM and others. They tried it and they liked it: orders of magnitude lower fees and orders of magnitude faster transactions. This experience will never let them come back to slow and expensive BTC for transfering value purposes. BTC has narrowed down all of its use cases to just store of value. Would you agree?