The former chancellor has been preparing to run for the leadership since before there was a vacancy, with his campaign website registered prior to the Prime Minister departing.
This perceived lack of loyalty could be a reason, but then it ignores the fact the foreign secretary has also been organising a campaign for some time.
The only difference really is that when it came to actually ousting Boris Johnson, Mr Sunak resigned, citing it as a matter of principle.
Unfortunately for him, those principles appear to be perceived as more calculating than honourable by a membership who perhaps value loyalty more than having a Prime Minister who could win.
If his resignation wasn’t enough of a problem, Parliament’s richest MP made things even worse by offering practical policies, rather than a vision people could believe in.
Mr Sunak has treated this race like he’s trying to win votes as Prime Minister, when it’s actually about convincing Tory members.
Warnings of fiscal sensibility might seem like traditional Conservative values, but speaking about ruling out immediate tax cuts now looks too negative.
It was exactly the same problem holding Jeremy Hunt back in 2019. Members don’t want explanations of why something can’t happen, they just expect immediate results.
With Mr Johnson, this became the norm as the deals others would never sign off on were approved and sold as gold by virtue of personality alone.
Ms Truss knows this and has cannily dismissed questions over her own financial plans as “project fear”, because debate is about slogans, actually.
Losing on the economics front, Mr Sunak has now naturally tried to outflank his former Cabinet colleague by promising a series of his own tax breaks.
The problem is, having spent so long insisting they were a “fairytale” and irresponsible, they can now be framed as desperate, or worse, an endorsement of Ms Truss’s arguments.
Mr Sunak has a slick campaign, but it’s been trying to appeal to the wrong people.
Polling shows he is the Tories best chance of winning the next election, but in skipping to the general public Mr Sunak may never get the chance to contest one.