There were a lot of different factors at work in this -
1. The "Enlightenment" distaste for revealed religion of any sort, thus any miracle must be pagan, not our nice rational Church of England.
2. The protestant distaste for Catholicism's local, physical associations and a desire to stamp out devotions to the saints as "a fond thing, vainly invented" led to a disavowal of the site of any miracle. This is why Walsingham, for example, was razed to the ground and the ancient statue of Our Lady formally burned in London.
3. The "cool" factor - pagans are SO much cooler than plain old ordinary Christians!
4. It sells, tourists eat it up. See No. 3.
5. Historical ignorance. Surprisingly prevalent.
Bigg red, the above, #37, may interest you.