Alton didn't have Merlin behind it, but was still much better known than Drayton even then. Nemesis and The Big One got a lot more attention than Shockwave, but not a massive amount of that was down to paid advertising. They were appearing on the likes of Blue Peter, for example, but it was because the parks were already well known enough to be familiar.
Now, Alton doesn't need a high quality product since Merlin will already factor in millions for advertising. Drayton can't do that, so even if they have a better product, it won't get the attention.
Also, I'm not sure that back in the early 90s that the parks would know to the same extent that they do now what each other were doing. The days of the big reveal are over, and everyone knows years in advance what everyone else is up to.