Because it sucks as a player. Even with Perian, QT7 is slower than MPlayerX, Movist and VLC. Try playing any WMV file with Flip4Mac installed for example. Furthermore, there will always be the odd file that only VLC can play.
Slower in what sense? I open a HD movie with QT7+Perian and it opens and plays as soon as I press the space bar.
And Perian plays:
MS-MPEG4 v1 & v2
DivX
3ivx
H.264
Flash Video
Flash Screen Video
On2 VP3, VP6, VP8
H263I
HuffYUV and ffvhuff
Indeo 2, 3 & 5
MPEG-1, 2 & 4 Video (in supported formats)
Fraps (up to v4)
Windows Media Audio v1 & v2
Flash ADPCM
Xiph Vorbis (in Matroska)
MPEG Layer I and II audio
DTS Coherent Acoustics audio
Snow wavelet video
DosBox video
Nellymoser ASAO audio
libavformat, from the ffmpeg project. along with AVIImporter.component:
AVI file format
FLV file format
NUV file format
libmatroska, along with matroska-qt.component:
MKV file format
Subtitles:
(Advanced) SubStation Alpha
SRT
SAMI
VobSub
liba52, via A52Codec:
AC3 audio
So getting VLC because you might play a file that isn't on this list hardly seems like a good justification.