Any idea of the reason behinds why 1994 is no longer the expensive one and 1998 is?
The supply of 1998 panda coins in general is relatively tighter compared to other years. This applies to the gold coins, 1 oz. coins, and the 1/2 oz. coin. Like Exchange said, people did not seem to know that it existed so you can still get them for a relatively lower price. However, I don't think this will last long.
It was only around two months ago when I saw Xu Hong list a 1995 1/2 oz. panda (small date) from Shenyang for less than $60. Although I was aware that it was rare, I passed on it for personal reasons. I have a few 1/2 oz. pandas but I'm concentrating more on other coins because although I like their design, they are too small for me. I prefer larger sized coins (at least 38mm in diameter). If I did not see other coins I wanted to buy at that time, I would have bought the 1995 1/2 oz from Xu Hong. If you're a fan of the 1/2 oz. panda, I suggest that you look for them before the prices go up. The same goes for 1998.