The immediate answer is that there is already a viable and attractive range of 20mm Sci Fi in the form of Rolf Hedges' Little Green Men Miniatures. The lack of a professional webstore alone prevents this from being a credible mass market option just now.
Whilst planning next week's game, Masterchef and I were discussing the state of play regarding 15mm Sci Fi and where the tipping point came that brought it out of a 30 year dark age.
That point was when Jon at GZG began releasing the first of his Sci Fi vehicles and AFVs, followed by a sustained onslaught of vehicles and support weapons. This was the quake that began the tsunami we are currently riding.
20mm has a lot to offer Sci Fi - loads of 'modern' kits to kit-bash, plenty of good Ultra-modern forces from a range of companies many of whom can be used as is - Britannia's SAS etc - others just need a visor or a closed helmet.
Camouflage and fancy Sci Fi paint jobs show up better in 20mm, you can use the terrain you've already built for Afghanistan or wherever and don't knock kitbashing either. When I started this blog back in early 2009 I was still kitbashing in 15mm. And some 15mm weapons, accessories and aliens (eg Khurasan's Ti Mohnid) will even bridge the 20mm divide as 15mm edges closer to 17/18mm anyway.
Rolf has told me that he is prepared to extend the Little Green Miniatures line and I encourage you to in turn encourage him if you want to see it happen. Send him your ideas and suggestions for extending the figure lines currently available and for creating new ones.
I think there is opportunity now, just as their was in 2008/9 for a manufacturer to establish a 20mm Sci Fi footprint and capture the 20mm Modern gamers who may want to expand their 20mm modern gaming, using the adobe or jungle huts etc they have rather than starting in a whole new scale.
Will this compete with the 15mm Sci Fi market? Yes, but only in a small way. The range of manufacturers and what they have to offer in 15mm will be hard to beat and will still tempt gamers to duplicate 20s in 15mm for many years to come. But for 20mm gamers who just want a taste of Sci Fi action on the ground between patrols or raiding forces 20mm Sci Fi will be ideal.