Or perhaps because historical monarchies didn't have the same level of technology that modern liberal democracies enjoy?

At the given time, they were replaced by democracies with the same technology level they had.

That response is nonsense, an unfair reading. Jaime already offered your hypothesis immediately preceding:

Is it because monarchies have had a tendency to turn into democracies?

He explicitly says that means something completely different.

I imagine that he means, quite correctly, that many comparisons between democracies and monarchies fail to compare examples at the same technology level.

As to the other point, I doubt Jaime thinks that monarchies turning into democracies is a very good argument in favor of democracies, just that it is a common implicit argument. I doubt that there are many people who think that monarchy is a good form of government at two technological levels, separated by democracy. Generally people who condemn democracy think that it was a mistake, perhaps historically contingent, or perhaps a natural tendency of technology, but one to be fought. Some reactionaries hold that this is a good time to pursue non-democracies, but usually because democracy is finally self-destructing, not because technological pressures have reversed course.

But monarchies turning into democracies is evidence against the stability of monarchies, and some reactionaries do implicitly make the argument that technology favors monarchy in two different periods.