One requires real money (recurring subscription) and the other (game time) does not.
You can conclude on your own whether/why Blizzard would want to incentive real money transactions from players over game token purchases.
It shouldn’t be difficult to surmise why they switched to the new requirement of a subscription for the Sylvarian Dreamer mount. Players were upset about it: Sylverian Dreamer Rant
That’s just the first thread that popped up, but you can find several Blue posts explaining the change. They didn’t say, “oh, we did it to increase our subscriptions” but that is the only difference between how the Dreadwake mount was acquired compared to the Sylverian Dreamer.
And, yes, players colloquially refer to their game time as “being subbed” to the game but I provided two distinct links to the two different game time forms so you can see Blizzard distinguishes between the two and for what should be obvious reasons token purchases are reported differently in their financials from real money purchases.