Why does the future always have to be liberal?

[TJ in Oregon]

Because American conservatives have spent their energy on blaming liberals rather than liberalism, and haven't tried to establish a positive alternative to liberalism, particularly as a fundamental aspect of how we see the world rather than as a purely political movement. Instead, American conservatism has simply responded to liberalism rather than building institutions on their own terms. American conservatism has tried to also frame all things in terms of rights and freedoms, essentially on liberal terms. You're not going to beat the liberals on liberal terms.

[TJ in Oregon]

Liberals want progress; conservatives do not.

Without liberal thinking, we'd still be owning slaves.  Every cultural and scientific advance is due to liberal thought.  Naturally, as we move farther into the future, the more liberal we as a society will become.

Conservatism will always be something of the past because they always look to the past (or at the very least, staying the same).  There is no push forward with conservative thought; it is always a look behind.  Make America Great Again implies that the progresses and advances we've made have not made us greater and that we should return to a prior cultural/scientific era (the 1950s, 60s, or the 1880s--that part is left open to interpretation).

Liberals look forward to the future; conservatives are frightened by it, which is why they resist almost everything that moves society forward.

Liberals accept the past, including past mistakes; conservatives long for the past, including those mistakes (which they don't necessarily view as mistakes).

This isn't a Republican vs. Democrat debate; it's an ideological debate and it's a fairly simple one.  The future is always going to be liberal because we will always be moving forward.  Even when conservative thought slows us down (as it has right now), we will eventually get back on track.  It's a hiccup that eventually stops so you can go on about your day; but you will go on.  Just as after the Trump era, American society (culturally, scientifically, technologically) will again move forward; but as long as Trump and his ilk are making policy, progress and societal growth will be retarded.

If you define liberalism as merely referring to whatever direction we happen to be heading, then of course the future will be liberal. If you define liberalism as a particular ideology about the nature of man how a society ought to be constructed, then it's not at all apparent that the distant future adopt that view as opposed to some other view. If anything, liberalism has taken a huge beating over the last couple years, although it is still the massively dominant ideology of the US and western Europe.

Also, the connection between liberalism and scientific advancement isn't particularly strong. If you look at the political views of influential scientists over the course of the last couple millennia, you'll find a wide variety more or less corresponding to the dominant ideologies in the cultures in which they lived.