Games are wont to end as trilogies. Going by Gears of War, Mass Effect, Halo and countless others, you could say it's an unwritten law. Much like The Dude, Insomniac's PlayStation-exclusive Resistance franchise will abide, with CEO Ted Price informing VG247 that the developer "won't be making any more Resistances".
This, by the way, is total fine by me, as Insomniac's currently working on its co-op shooter Overstrike. If there's anything I love, it's co-op games -- as long as they can handle mixing narrative with gameplay. Considering the second player in Resistance and Resistance 3's co-op modes is essentially ignored, I don't see it being an issue.
Despite having sold 385,000 copies in the same time it's taken Resistance 3 to hit 180,000, I skipped over Resistance 2 due to its lack of a proper co-op campaign. I did however play the third game from start to finish with my brother in tow and while it was enjoyable, I'm having trouble remembering any stand out experiences.
It also had its fair share of bugs -- during co-op, one of us would often get caught on the wrong side of the transition doors that split up levels and scripted NPCs had a habit of halting mid-mission, forcing us to reload from the last checkpoint.
To summarise, I got the impression playing Resistance 3 that Insomniac had taken the series as far as it could go... perhaps a bit too far. I say let it rest.
Update: Ted Price posted a video message explaining the decision (thanks NotoriousR). Basically, the studio feels the story has reached its natural conclusion, as far as it's concerned, but that doesn't rule out it being taken further by other developers.