MrSID is no longer being supported in ArcGIS 10 as an export format. So, unfortunately, that meant it had to be pulled from ArcPad's Data Manager too.
Don't be mistaken. All your existing MrSID imagery will still continue to work. The MrSID reader capability exists in both ArcGIS 10 and ArcPad 10.
I won't even attempt to offer a guess as to why the MrSID writer was removed from ArcGIS 10.
I acknowledge that it's you the users that suffer whenever previous functionality gets deprecated. ArcPad Data Manager, unfortunately, gets impacted since it has to reflect that loss of functionality from ArcGIS 10.