In theory, I suppose, it is because political leaders are accountable for carrying out their manifestos - manifestos which are published, written and negotiated by the party, not the government. In that sense, it's reasonable for the party to have a delegate on the cabinet.
In practice, though, we all know that's not what the position is for. It's a channel for lobbying by higher-ups in each party,which is the principal reason anyone donates significant sums to them.