It's quite simple really.
To make very accurate preds and aliens you need a few factors to click into place: time (which costs money) for research, talent and to be a big fan of the creatures.
Time and money are HT's call, talent and fandom are the sculptor's. Personally, I think HT (i.e. Howard Chang) doesn't really give much thought to the accuracy of these creatures, and since people buy them anyway there's no real incentive to spend the money and effort to make them really accurate. As for Tsang's talent and fandom, I'm quite sure he's a talented sculptor, but he clearly does not have a great interest in these creatures, otherwise the preds and aliens would certainly have been a lot more accurate from the beginning.
For a big fan of the aliens, and to a lesser degree of the preds, it is sad to see a company as resourceful and innovative as HT just squander the opportunity to make absolutely killer figures. The figures are great, I own a bunch of them, but it's quite apparent that HT just don't put that much effort into making them.