According to Wikipedia, SCART has pins for "Component Y", "Component Pb" and "Component Pr", which (I think) can make a HD YPbPr component video signal.
Why can't SCART carry a HD video signal if it is capable of carrying a YPbPr video signal?
HD is more that just the fact that it uses YPbPr components.
SCART defines signal speeds that both endpoints must support in order to be compatible, it does this by specifying cable types, contacts and electrical specifications such as voltages used. This makes for a very specific range of signals that be sent over the wires. Put simply the signal speeds supported by SCART devices very firmly limit it to SD content.
HD content requires much faster signalling rates than SCART supports due the types of transceivers present in SCART compatible devices.
Devices capable of "reading" HD YPbPr signals will almost certainly not be SCART compatible.
AV is anoluge, YPbPr is analogue, SCART is analogue, SCART can do AV, there's no reason it couldn't do YPbPr if both ends supported it.
YPbPr is Analogue HD, HDMI is Digital HD
Analogue and Digital HD are incompatible. SCART could never do HDMI, YPbPr could never do HDMI. It has nothing to do with cable speeds.