Bethesda probably intended that FO3 would not be like FO and FO2, but rather their own spin on the world of Fallout. As the T-51b armor was the iconic Power Armor of Fallout, it had to go (for the most part) to help represent this fact- basically, a new iconic power armor for the reboot of the series. After all, before there was FO3, there WAS no such thing as the T-45d Power Armor.
I think that they gave it no mind initially... as the sprites were 70x 29 pixels or so, and 'nobody would really care'; I think the game was likely mostly in place when they added a closer version that better matched the original.
Nothing in FO3 looks like it did in FO1 or FO2 ~which is a shame IMO because its a game heavily based around its own history; and full of antiques. The engine doesn't care, it could actually run the close up head graphics as the realtime game assets and the weapons and item would be no more difficult to model new, than to model familiar. :shrug:
http://i271.photobucket.com/albums/jj125/Gizmojunk/mr-handy2-1-1.jpg was http://i271.photobucket.com/albums/jj125/Gizmojunk/Mr-Handy31.jpg, and was featured in the first game's opening
title ~yet they made it completely different, and named it the
same (even though that's not what it looked like, and the the fans would notice). The vast majority of players would be none the wiser, for having never played the series before (but series fans would notice).
**Edit: That Mr.Handy model in action (animation test).
http://www.youtube.com/watch?v=UUslVGxW_WE