Because they understand that this Labour Party is an anti-military interventionist party, and that makes the Tories - even a defence budget axe-weilding Tory Party a more attractive proposition.
There is a class element to this as well. Lots of ordinary squaddies do like Labour but the higher up the ranks you go the more antagonistic they are. However if you continually pump the idea that Corbyn was in the IRA to enough people then eventually some will start to respond to that message.
We have to look far more closely at how we utilise our armed forces and where they are utilised. There has to be more done to tackle the pervading culture within the military andparticualrly the terroristic doctrine of fascism. Finally the Irish struggle needs to be taught far more objectively from Britain, and that can start with explaining that Sinn Fein is a political party, much akin to the DUP who have been put in government. The only difference that the knuckle draggers within Sinn Fein (the anti-gay, anti-abortion bigots) are very much a minority, whereas they are the central tenant of the DUP.
What is not on though, is to make out one side are terrorists and the other should be invited into government as our long term friends. Theresa May should face arrest for that little barb.