rationale of light travelling in straight line?

I do not know whether you understand the meaning of the word rationale? If you mean evidence then evidence then there are evidences of rectilinear propagation of light like formation of sharp shadows of objects having the same outline of shape. Ans also there are evidences in the phenomenon of interference and diffraction that light does not travel in straight line but it bends round the corners just like any wave.

When the obstacles in light are of the dimensions much larger than the wavelength of light if we consider them as waves, then light seems to propagate in straight line. But when it is not so then it bends round the corners. So the rationale for approximate travel of light in straight line is explained by the Fresnel theory of half period zones. The whole wave front is imagined to be divided in the regions called zones such that successive zones send waves of opposite phase. If the obstacle is big enough to hide the first few tens of zones, then it can be shown that the efefct of remaining zones in producing the enevelop of the seconday waves is almost zero. So we find that light travels in straight line approximately.

Now rationale for light to travel in straight line if they are considered as particles as in Newton's corpuscular theory is that the particles are so 'light' (not heavy) and move so fast that the curvature produced in motion due to gravitational effect of surrounding objects is too small to be noticed. Even if we consider photon model of light the relativstic effect of bending of light can be made observable if the massive object is large enough like sun or galaxy, and light crosses it at very small distance.

Light would travel in a straight line, but it's course is changed by gravity. Its path is "bent" slightly when passing near a massive object (like a star). The only light traveling in a straight line is the "first" light produced during the "big bang" and that is because 100% of the universe's mass is behind it and it is not bent by passing near anything.