Why does a hybrid with 700x42 ride smoother than a front suspension mountain bike with 26x1.75. Both tires are smooth street tires. The hybrid tires are slightly narrower and higher pressure so I thought it should be a bit harder ride.
The suspension does absorb some of the shock, but the rear wheel has no suspension. The hybrid is rigid, but overall the hybrid seems to ride smoother. Only on the bumpiest roads or terrain do I prefer the suspension over the rigid bike, but to be honest, the suspension doesn't seem to help all that much. It does dampen the ride on such terrain but I'm not in those situations enough that I prefer to take the squishy bike over the rigid one for city rides.
Is there more to this than just the suspension and tire size? Both are aluminum bikes except the mountain bike is suspension fork and the hybrid has a carbon fiber fork.
Ummm ... (overall) wheel diameter would be contributing pretty much all of the difference (what isn't contributed by possible differences in tire quality/construction).
As you've specced it above, your wheel diameter on your mtb is about 25.5 inches; on your hybrid about 27.8 inches. That results in a very large -- and very noticeable -- reduction in 'angle of attack' in favour of the hybrid. When each wheel hits a bump (especially a sharpish one), or encounters a dip, in the road surface, the larger wheel will go up and over more easily, or not fall in as far (and again, roll out more easily once it has fallen in), as the significantly smaller wheel. Lower rolling resistance (in this sense) and a smoother ride are the result.
My 700c Sirrus w/32mm Paselas is smoother riding, in this way, than my 26er mtb w/1.6" (40mm) road slicks, even with the higher pressure required in the former.
It's the whole principle behind "29er" (and now 650B) mtbs.
The front suspension on the mtb isn't really intended to compensate for this kind of difference, nor (as you've found) does it do so.
I would put it up to geometry and fork angle as well.
Also, if you'll measure the thickness of the respective tires and do some math you'll probably find more volume to work to the hybrid's advantage. Generally, 28" tires have the volume of a 26" tire one size thicker (eg. 28x1.60 ~= 26x1.75).