1) You can play any GBA game on your Gamecube using the GBA player
(altho some of them won't play very well, or will be very difficult to
play well; like Boktai, which needs UV light). If you view a GBA game
and a GC game as interchangeable, you can play the GBA games on your
Gamecube for about $50.
2) Very few GBA games are of the type that people would accept on the
Gamecube as a game made for the GC (or perhaps that Nintendo is willing
to accept people will like on the Gamecube). There are exceptions, like
puzzle games, some retro games, some tactical games.
3) GBA games aren't really directly portable to the Gamecube (or if they
are, no one or almost no one is doing so).
4) Even if you feel a game for the GBA is interchangeable with a game
for the GC, that is not at all true for programmers.
5) The GBA has a much larger installed base than the GC, leading to more
potential customers than a port to the GC. The average GBA game also
generally has a lower development cost than the average GC game (the guy
who made Rez recently said he could develop a GBA game for around
$70,000, while a console game would cost around $3,000,000.

See 5 above. You need a relatively small staff to make a GBA game, as
opposed to console appropriate (nowadays) games which require a massive
art department to make big 3D environments, sound guys, more complicated
physics, lots more possibilities to test, etc.

You do realize you can own more than one console, right?

Just a thought but it's probably easier to make games for the GBA than
for GC.  Also more people have GBA than GC.

Games are cheaper to develop for the GBA.

--
-Daniel "Mr. Brevity" Kolle; 16                        A.A. #2035
Koji Kondo, Yo-Yo Ma, Gustav Mahler, and Krzysztof Penderecki are my Gods.
Head of EAC Denial Department and Madly Insane Scientist.

becuase they know GameCube will not make them a profit, why? becuase not
many people like the gamecube, only a few that are attached to mario