Why do people prefer to buy Bitcoin over other cryptocurrencies?
There are many reasons:
Bitcoin is the most capitalized market amongst cryptocurrencies, and that is an important measure for any form of money. If it is difficult to trade a form of money without crashing the price of it, then it is not a very useful money in the first place! As liquidity grows, more economic activity can occur using BTC (liquidity begets liquidity), it is a positive feedback mechanism.
Systems of money benefit from network effects. It is beneficial to own a form of money that a large number of people accept, compared to a specialized money that only a few people will accept. Bitcoin is still 'specialized money' in the big picture, but it is certainly the most widely accepted cryptocurrency at this time, with the largest userbase.
A decentralized network
When compared to a centralized financial network, the operation of the Bitcoin network is quite inefficient: every network node independently computes and validates the entire history of the network. This inefficiency is seen as acceptable because it results in a network with no hierarchy amongst nodes: nobody is in control of the network. There is no central admin. There is nobody that can exert undue influence on the network (for example, to change the rules, or inflate the supply). This point is very important! Many other cryptocurrencies are tied to some central authority, and this represents a single point of failure, that a user of that currency must trust to not be abused or attacked.
Historically, the Bitcoin network has proven to be extremely difficult to change (as evidenced by events such as the 'Segwit2X' debacle), which gives investors confidence that the value they store in the network will remain secure (1 BTC continues to = 1 BTC). To contrast, the Ethereum network regularly enacts hard-forks that change the rules of the network (see: the DAO attack, ice-age setbacks, changes in issuance of new ETH, etc).
The history of the Bitcoin network is probabilistically secured through the economic incentives of Bitcoin mining. To put it more clearly: each new block on the network provides a reward which incentivizes miners to spend resources mining and progressing the network's state. To alter the history, an attacker would have to spend more resources than the entire rest of the network combined (this is called a majority attack).
As a user, it is thus in your best interest to use the network which has the highest amount of security, so that there is the lowest chance of someone successfully attacking the network you've invested in. To date, Bitcoin has always had the largest amount of security, no altcoin has come close to it in this regard.
As mentioned above, the rules of the network are extremely hard to change. Making a change requires that a super-majority of users will all agree to the change, and then update the code of their nodes to reflect the desired change. Because of this, the issuance and supply of BTC is very well understood, extremely predictable, and likely almost impossible to alter. This means that investors can have confidence that they understand their stake in the network, and this stake will not be changed without notice (by, for example, a central authority deciding to alter the supply).
With all of the above in mind, the result is a network that grants the user financial sovereignty: un-censorable money that is completely under the control of the user. Other cryptocurrencies do not have the level of decentralization required to strongly guarantee such a thing, many of them are in fact entirely centralized, depending on some central authority to guarantee the value, services, security, or function of the network.
The Bitcoin network has been around for the longest time, and as a result of this, it has a large developer base working on a huge variety of projects that will benefit the network and it's users. Since the code is open source, you can view the development progress in real time by checking out the Bitcoin-core project's github repository, the bitcoin-dev mailing list, #bitcoin-dev IRC channel, etc. Beyond this, you can also check out development of the Lightning network (LND, c-lightning, etc), the lightning-dev mailing list, the many wallet projects in the ecosystem, and the many systems and services being built to utilize Bitcoin.
Following the work can be dizzying, but the results are clear: as time has passed it has become increasingly easier to use and interact with the Bitcoin network. It seems logical to conclude that this trend will continue.
You may have noticed that everything written above is largely a social phenomenon: the code itself is entirely open-source: you can copy/paste all of it, tweak a few parameters, and start up your own altcoin network without much work at all. What you can't copy/paste is the network of humans that partake, and that is where the real value comes from.
If Libra is introduced as a stablecoin with a company like Facebook supporting it, why would there be a demand for BTC?
Libra is no different than any other altcoin that has a centralized point of failure, posing a huge risk to its users. Further, facebook's abhorrent privacy policies and past abuses of user data make it inconceivable that a currency they control would gain widespread trust.
Why is there still demand today when Tether can serve as a stable medium of transaction?
Similar to Libra, Tether is backed/guaranteed by a central authority, it does not promise the same things that Bitcoin does (which isn't to say these things can't have use-cases in their own right, but the key point is that they are not equivalent to Bitcoin).