Paul Ryan is the Speaker of the House of Representatives, not the head of the Republican Party. Donald Trump is running to be the defacto head of the Republican Party and will be should he be elected to the Presidency.
Obama is head of the Democrat Party by reason of being the sitting President. Debby Blabbermouth Shultz runs the DNC, but Obama runs the overall Party.
Ryan already has Constitutional responsibilities that transcend the political race for the Presidency. I understand why he would be uncomfortable with an outright endorsement of Trump. I would wager most of the voting electorate will be holding their noses to vote (once again) for a candidate that is far from their favorite, yet still the lesser of two weevils...
Why should Speaker Ryan not be allowed to show a firm reluctance for someone who is so far outside the norm as to not even clearly understand who the Speaker is and why he is Constitutionally separate in many ways from the Presidency??
Of course we live in an age where people really think Bruce Jenner is a frackin' woman...