You add it to VAT to make the admin and collection easy. Do you have any idea what it would cost to legislate for and administer a new tax? It's obviously massively simpler to amend one section in one act than to bring in a whole new tax.
It could have been increase in VAT specifically to pay for xyz or the 2.5% spelt out. Just "increasing VAT" without saying exactly what for makes it very easy to continue. Good example is the ozzie flood levy....
A whole new tax would be messy indeed.