Why only at night? Why unobstructed in the daytime only?

Radio waves radiate more or less in all directions from an AM tower, meaning
that some of them radiate skyward.

The ionosphere, in the presence of solar radiation (i.e., during the day),
absorbs the radio waves. In the absence of solar radiation (i.e., during the
night), it becomes a reflector, bouncing the signals back to earth hundreds or
thousands of miles beyond the transmitting tower. This can create interference
with stations on the same or adjacent frequencies at night, where no such
interference would occur during the day. To avoid such interference, many
stations must either cut transmitter power at night, or employ directional
antennas...or sometimes both.

What is a skywave?

Radio waves reflected by the ionosphere under nighttime conditions.

What is a clear channel station?

It's now an obsolete term, but "clear channels" were AM frequencies with only
one station on them. What are now referred to as "clear channel" stations (not
to be confused with Clear Channel Communications, the group station owner) are
for the most part the 50-kW AM stations using non-directional antennas, many of
which have been around since AM radio's early days of the 1920's.

Why do they have to protect other stations?

The AM band is organized as "first come, first served." Stations that were
broadcasting first get priority, and are protected from interference by
stations that came later, through the use of lower power or directional
antennas.

why only at night?

See above, re the changes in the ionosphere depending on whether or not the sun
is up.

Why do newer stations protect older ones?

FCC rules, plus international treaties to which the United States is a
signatory.