That's not entirely true. I've seen and continue to see lots of games announced for PC and PS4 and not XB1 from indies (and a few PC+XB1 and not PS4). The reasons for that might not have to do with install base but it can't be assumed that all platforms are going to get all games.
edit: another two announced as such today on EG
http://www.eurogamer.net/articles/2015-06-12-zeno-clash-dev-announces-the-deadly-tower-of-monsters
http://www.eurogamer.net/articles/2015-06-12-tales-of-zestiria-to-get-western-release-this-october
It's definitely a lot more complicated than install base.
With indie developers it's going to be more about limited development funds combined with which platform holder is giving them a better deal and/or support with porting their game to their platform. Usually, it'll eventually come to whatever platform didn't get it first. Assuming the title was successful enough to warrant expending funds on another port. If not then it likely wasn't financially successful on the first console it launched on and thus porting to a second console might not be justified. Or it could be that in exchange for assistance in porting to the first platform, they were required to sign an exclusivity deal.
For example, Warframe (incredibly successful indie PC F2P FPS) launched first on PS4 but was then later ported to XBO as well. On the other hand Smite (incredibly successful indie PC F2P MOBA) is XBO exclusive for now, but will likely get a PS4 port at some point in the future. Those are easy as they are large enough and successful enough that it's not a question of if it will eventually be ported to another platform, but when.
For smaller indie titles, that becomes much less certain. If the developer is small enough porting to one console or the other could represent a significant risk. If that doesn't result in enough of a success to validate that risk, it's unlikely they'll take the risk in porting to another console.
It's rare to have an indie developer that is so big and so successful that they can afford to port to both consoles (or more as some target Wii-U, 3DS, and occasionally VITA as well) simultaneously. Usually it'll be one console platform followed by another then another, etc.
PS4 having the larger install base means that it's more likely to be the first targetted console. With XBO tagged as getting a port ASAP, if possible. But as Smite shows above, larger install base isn't always the determining factor.
Regards,
SB
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