~It's the only Secret Rare card in the BW sets that does not have a normal variation (all the other shiny cards were printed in other sets, but Rayquaza was in Dragon Selection, which we don't have yet)
~It's capable of leading and donking basic Dragon types, doing 40 for only a single {L} and milling your top 2, plus its second attack is {L}{R}{C} for a guaranteed 90+pierce all (which can also OHKO any dragon)
~It combos well with Eel decks, as does Rayquaza EX, but can also keep ahead in the prize trade
~and, for the lolz, you can search it with Heavy Ball
Its price is declining because it can only really be used in Eel decks, plus we're now past the street release date for Dragons Exalted, but still people need to pay a pretty penny to get one.