The reason is quite simple and is shown by an analogy.
Supposing that there was an election for the position of supreme Dictator, and the two candidates were one who was the usual politically correct Leftist.
And the other was someone who claimed to be on the Right, and opposed to Leftism (was fully of mockery and loathing for Leftism) - and he was who (like many or most mainstream secular Right politicians and theorists) stated that he was sympathetic and supportive towards Christianity - but who was Secular in the sense that he did not put religion first, but put politics first.
Suppose that (as usual) this mainstream Secular Right candidate was secular in that all his policies ignored any justification in terms of what was necessary to God's wishes or will, or the divinely-ordained order; that the policies did not derive from spiritual justifications - that is to say, his economic, social, legal and all other policies were justified in terms of the same 'utilitarian' criteria as the Leftist candidate (which is making most people happier or reducing suffering (in this life and this world); or making them safer or more secure; or being for justice or against injustice (in some vague and undefined fashion); or more rational, or more effective, or more efficient)...
Well, what are we to make of this putative Secular Right candidate's stated sympathy towards and support of Christianity?
Well, what he is saying is - first beat the Left, then we can talk about religion. He is saying: first give me power, and religion will come soon afterwards.
He is saying: "I am not a Christian now, because obviously it would be unreasonable/ inefficient/ counter-productive for me to be a Christian already; but trust me - very soon after I have obtained power I will become a Christian - or the next best thing."
He is saying: "Put-off conversion until after we have sorted out the constitution/ economy/ national security/ the police/ the legal system/ education/ health care/ the mass media... and as soon as these are sorted then we shall immediately spring into vigorous action and support Christianity."
He is saying: "Men's hearts do not matter. It is not necessary for the nation to repent and awaken to Christ - because with proper policies, we can get the benefits of being Christian without actually being Christian.
He is saying: "There is no need for Men to be Good if the system is good."
I think it is obvious what any serious Christian would think about such a candidate for the Dictatorship.
But what is true for a person is true for a party or a movement: if they are not Christian now, then they are not Christian at all. And if they are not Christian then it does not matter what they say (or even what they believe) - under the conditions of modern politics - if they actually get power, then they are going to be anti-Christian.
And since, if Christianity is true, it is the most important thing - and if it is not the most important thing then it will surely be opposed - then the Politically Correct Left and the Secular Right are Tweedledum and Tweedledee.
Words are cheap, 'promises' can be bent or broken, vague good intentions are meaningless - because when all your priorities are secular, you are de facto anti-religious; whether you claim to be on the Right, or not.