It’s simple : HTC (or any other handset maker) didn’t publish any API (Application Programming Interface) to make scrollable widgets.
They reserve this feature for their own widgets.
No API means impossible for developpers to create scrollable widgets for those Home.
We defined a new open source API to let developpers create a scrollable widget for any alternative Launcher (already supported by ADW Launcher, Zeam and Launcher Pro).
The source code is available, so, any alternative Home can add scrollable widgets support.
I don’t think the situation will change until Google add this support directly in Android (3.0?).