Part of the problem there is in the nature of each sides campaign. The most common way to build an argument is to weigh up the pro's and con's, the upsides and downsides. The "Yes" campaign can promise the world on the upside, say that it'll all be amazing in the future, simply becuase nobody really knows what will happen.
Whereas the "No" campaign can only say the upside is it will be exactly the same as today, just the boring status quo. So they've been kinda forced into negative campaigning just by the nature of the decision to be made.
They could've tried some bribery perhaps, "Hey, we'll invest billions more in Scotland", but that would be really transparent on both sides and probably lose them more than they'd gain.