Why do none of the classical accounts of the British campaigns mention the druids until Suetonius' attack on the isle of Mona? And why didn't the Welsh druids head north of Hadrian's Wall?

The Isle of Mona - Ynys Mon or Anglesey, Wales - is often called the druids' last stand. Suetonius Paulinus led the invasion of Wales in 60 AD. Roman historian Tacitus describes in detail his brutal attack on Mona:

xxix: [Suetonius Paulinus] prepared accordingly to attack the island of Mona, which had a considerable population of its own, while serving as a haven for refugees; and, in view of the shallow and variable channel, constructed a flotilla of boats with flat bottoms. By this method the infantry crossed; the cavalry, who followed, did so by fording or, in deeper water, by swimming at the side of their horses.

xxx: On the beach stood the adverse array, a serried mass of arms and men, with women flitting between the ranks. In the style of Furies, in robes of deathly black and with dishevelled hair, they brandished their torches; while a circle of Druids, lifting their hands to heaven and showering imprecations, struck the troops with such an awe at the extraordinary spectacle that, as though their limbs were paralysed, they exposed their bodies to wounds without an attempt at movement.

Then, reassured by their general, and inciting each other never to flinch before a band of females and fanatics, they charged behind the standards, cut down all who met them, and enveloped the enemy in his own flames. The next step was to install a garrison among the conquered population,² and to demolish the groves consecrated to their savage cults: for they considered it a pious duty to slake the altars with captive blood and to consult their deities by means of human entrails.

While he was thus occupied, the sudden revolt of the province was announced to Suetonius.
- Annals XIV.xxix-xxx.

Suetonius had to drop his war on the Mona druids to deal with the Boudicca rebellion. The Romans didn't stop there, though. Tacitus describes a second attack on Mona by Gnaeus Julius Agricola in 78 AD:

He almost exterminated the whole tribe: then, recognising the necessity of confirming first impressions, knowing that he depended on the issue of his first campaign to terrorise the enemy for the future, he determined to reduce the island of Anglesey, from the capture of which, as I have before recorded, Paulinus had been recalled by the general rebellion in Britain.

His plans had been hastily formed and so, as was natural, he had no ships on the spot; yet the resourcefulness and determination of the general bridged the straits. For after unloading all the baggage he picked a body of native auxiliaries who knew the fords, and had that facility in swimming which belongs to their nation, and by means of which they can control simultaneously their own movements, their weapons, and their horses: he then launched them upon the enemy so suddenly that the astonished islanders, who looked for fleets of ships upon the sea, promptly came to the conclusion that nothing was hard and nothing invincible to men who fought in this fashion.

Accordingly they petitioned for peace and surrendered the island.
- Agricola 18.3-5.

Now if the druids were so troublesome, why were there no accounts of them in other parts of British Isles? And why was Mona their last stronghold? Surely the Celts above Hadrian's Wall would have welcomed the Welsh druids.

Web sources:

Channel4.com
Yns Mon: Last Outpost of the Druids

You can read more about the final invasion of Mona HERE