I am but a humble insurance clerk.
OK, so let's see what we do know....
(A) Allianz have already discharged their contractual duty under the OP's former policy, treating him (by his own admission) very fairly. That policy is now dead, the subject matter (one motorcycle) no longer being in the possession of the policy holder and him having indemnified fairly and in full.
(B) Allianz are at liberty not to invite new business or solicit renewal for any individual (or whole classes of business) as and if they see fit. How else did, for instance, NIG withdraw from underwriting the former BMW Motorrad and Honda schemes?
(C) When the OP sought a quote from Allianz for his new motorcycle they (and other insurers, so the OP tells us) now view him as:
Had pay out for my bike that was stolen, Now am trying to get new insurance but due to the bike been stolen I am now classed as HIGH RISK and cant get insurance.
He then went on to tell us:
.....now classed as high risk as bike was used for work
Have rang 15 companies today and as soon as you say bike stolen sorry your a high risk.
Mac have just emailed we can insure you but due to the value of the new bike/stolen bike we will need to be paid in full £3000!!!!
So that is Allianz and fifteen other insurance providers (16 in all) who are all telling the OP the same thing, that he is a high risk. He does however have one quote from Mac (whoever they are) for apparently an annual premium payable in full of three thousand pounds, so somebody was a least prepared to offer terms.
If we follow your chain of logic and assume, just for one moment, that Allianz are somehow under some sort of legal, moral, ethical or consumer lead obligation to insure a motorcycle just because they once insured another bike for a policy holder and paid him a total loss in the same year, or if they do just want to be even slightly helpful to their former customer... All they need to do is, like mac, quote northwards of £3000 right up to or higher than the value of the motorcycle. The third party liability section of the policy is unlimited, so they really can price it how they like. Of course the OP won't accept it. Or they can just refuse to quote and save wasting any more of their and their former customer's time, which is what the OP implies they did.
(D) Having drawn a blank with Allianz (who either refused to offer terms, or simply offered an annual premium too high, it matters not) and 15 other insurance providers and the combined might of UKGSer's 1200 GS section... The OP pressed on calling round and BINGO! One (out of 17 or maybe more companies) came up with a quote a shade under a grand, which the OP bought. Like the Lone Ranger riding over the bluff, the RAC (much maligned on these pages) saved the day.
Why did he buy it? We can only guess:
(i) He has bought and paid for his new motorcycle, which us sitting in someone's showroom. It can't sit there forever and he wants to ride it
(ii) He's run out of time and / or options
(iii) There is possibly a whole lot more to the story yet to be revealed.