I recently answered this question. While researching for the answer, I found one point quite counter-intuitive. The heavier hydrocarbons resides in stratosphere while methane, the lightest hydrocarbon is more abundant in troposphere which is considered the densest layer. Wikipedia says that heavier hydrocarbons like ethane, methane, diacetylene all condense in a narrow layer between 160 and 320 km in altitude which is the colder lower part of stratosphere forming haze layers while methane which is the most abundant hydrocarbon in Uranus reside in the troposphere. Shouldn't it be the other way around - methane in the upper atmosphere while the heavier and denser HCs all condensing in the lower part of atmosphere?
Methane is more abundant in the thermosphere and the stratosphere.
On the stratosphere, the Wikipedia article you cite says
The most abundant stratospheric hydrocarbons after methane...
Also note that having condensed clouds of a material at one height does not mean that this material is confined to that height.
A further point is that the atmosphere of Uranus is turbulent and mixed. Any vertical stratification in composition is unlikely to be determined just by molecular mass.
No knowledge on the subject, but I'd venture from looking through that article that it could well be the same reason lighter $O_2$ has peak concentrations in the troposphere on Earth, while heavier $O_3$ has peak concentrations in the stratosphere (the troposphere and stratosphere here having fairly similar temperatures) (though the stratosphere has a much higher $\theta$ (potential temperature)).
From that Wikipedia article:
Infrared spectroscopy, including measurements with Spitzer Space Telescope (SST), and UV occultation observations, found trace amounts of complex hydrocarbons in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by solar UV radiation. They include ethane ($C_2H_6$), acetylene ($C_2H_2$), methylacetylene ($CH_3C_2H$), diacetylene ($C_2HC_2H$).
And the ozone layer being in the stratosphere on Earth has the same basic explanation.
Now what that doesn't explain is why the heavier hydrocarbons don't wind up concentrating quite a bit in the troposphere as well -- precipitation/mixing/mass stratification all would seem to help increase concentration below. Ozone here is unstable so that its highest concentration stays in the stratosphere by far... perhaps the complex hydrocarbons likewise react away before they can accumulate in major concentrations below??