I'm pretty sure it's merely a coincidence.
Riot choose to make skins, most of the time, based on the opportunity for a really cool theme, that the team is passionate about, on a champion.
For (an over-exaggerated) example:
Let's say they have an amazing idea for an Ultimate skin for SORAKA- a champion with strong orientation towards the Support role.
The team is incredibly passionate about this idea. They feel like they're onto something big and awesome, involving unbelievable new tech that will make the frequency of "PLEASE UPDATE PULSEFIRE EZREAL" threads go up by 500%!
But then, they suddenly remember how the last four Ultimate skins were for a Bot-lane-oriented Marksman/ADC, a Jungle-oriented Juggernaut, a Support-oriented Enchantress and a Mid-laning Burst Mage.
And, you know, Soraka is a Support-oriented Enchantress as well...
So, let's say they'd then decide to start looking for a new champion- someone who usually goes Top- but they're still super passionate about that Soraka Ultimate skin idea.
They're making concepts, discussing, but none of the new ideas are as awesome as the Soraka one.
So, would the fact that there was no Top-laner amongst the current four Ultimate skins stop them from making another one for champions that are usually played in the other four positions? I'm almost certain it wouldn't. Especially if it's one that awesome. :P
######Obviously, this example might be/is over-exaggerated, as I previously mentioned, but those things are often what drives new skins- Passion and excitment, for the most part, so I had to show it.
######Also, it's not that I mind an Ultimate skin for a Top-laner, of course. If it's awesome, I'd actually be pretty excited for it, too.