The only reason that I can come up with is... that
impedance (200 ohms) is just what that specific
antenna type looks like. As you know, not all antennas
are 50 ohms driving impedance, infact most arent,
so you have to match to them. It just turns out
that some standard kinds of antennas, like monopoles
have a pretty low impedance to start with and can
be used in 50 ohm systems without matching. Some
patch antennas can look like 200 ohms due to the
physics of the antenna.
In other words,
there is no advantage to having a 200 ohm termination
impedance specifically, but the antenna impedance
is more driven by the type antenna and what kind
of performance you need from an antenna pattern
point of view. Matching is often secondary.
If you have a resistive 200 ohms, then consider
yourself lucky, that's an easy match.