Why did Young use a monochromatic source of light in his experiment? Why did Thomas Young use a monochromatic light to observe the phenomenon of interference in his double slit experiment?
 A: Different colours of light have different wavelengths, and as such some will diffract more than others. When you use many different colours the patterns of light that will be observed are superposed, and as such it can be hard to pinpoint a maximum, making the data harder to retrieve.
A: He wanted to be sure that he was observing a property of light itself, as opposed to an effect caused by a light source providing a cocktail of wavelengths. 
Light splits in a prism because the different wavelengths diffract differently through denser materials. 
monochromatic light (obviously) doesn't, because the wavelength is the same, so any interference patterns were down to the photons themselves, not the light source
A: First off, quoting Wikipedia, "Tony Rothman ... argues that there is no clear evidence that Young actually did the two-slit experiment."  The same page says the original description from Young involves sunlight.  Even if he did, the subjective reason of choice could be simply that he have blindly put every light available at hand behind the slits and found the monochromatic ones best for demonstration.
But if you are asking "why we use monochromatic lights today when doing the double-slit experiment", then it is certainly because that they produce easily measurable outcomes.
A: I think that the evidence is that at first Thomas Young did not use a monochromatic source (which he called homogeneous light) although of course using such a source would subsequently make the fringes easier to observe and obtain more accurate values for the wavelength of light.  
He investigated of sound and then thin film interference and it was from these experiments (and using the data obtained by Newton) that he was able to estimate the wavelength of light.  
He used sunlight to show diffraction due to a single narrow card as the Sun was a "handy" bright source of light although he was able to use candle light to observe the diffraction due to a strand of hair.  
In his Lecture 39 "On the Nature of Light and Colours" Young gives evidence for particles which have rotational and undulational (vibrational) properties and he mentions a beam of homogeneous (monochromatic?) light falling on a screen in which there are two very small holes or slits which produce equally spaced dark stripes and the middle of the pattern always being bright.
He then goes on to mention that for white (mixed as opposed to homogeneous) light the initial impression os of white light fringes but on closer inspection there was a "beautiful diversity of tints (colours)".   
One of Young's biographers writes "Frustratingly, he never published a systematic experimental paper using the two =-slit arrangement . . . " so what he actually did and used can only be found in his lectures and his notebooks.  
I have not been able to find a clear indication of what sources Young used to produce "homogeneous light"
