That's actually a good question.

It looks like Metroid haven't been able to increase sales from the "first" game in both of the times the series has started.
It looks like the competition have always beat Metroid, or atleast eaten away its sales to an extent.
Kid Icarus propably got some of Metroids sales, both were adventure games (admittedly, Kid Icarus may have been more like a platformer), but KI was more linear and accessible.
By the time of Super Metroid, there was competition from Super Turrican (and on the Megadrive side, Mega Turrican), a game that have been compared to Metroid a numerous times (despite being a sidescrolling shooter and therefore more accessible). This argument is supported by the decline in sales in Japan, where the series declines in a same fashion with the sales of the consoles, while in the west, where Turrican is much more recognised, the sales drop is huge (especially in others, where SNES outsold NES).
After the series being on a break for one gen, it returned to GC as in the form of Prime. Metroid Prime did by far the best, if we look at sales on a relative userbase (2,7M on 60M installbase vs. 2,8M on 22M installbase). Prime didn't have any real competition on GC, until Echoes, which suffered from being compatible only with 60Hz TV:s and having Resident Evil 4 having its launch soon after Echoes.
Corruptions sales were most likely killed by RE4 Wii Edition and REUC, both being "the" games that Corruption was supposed to be, as how to control shooting with Wii Remote and setting standards to it. Also, it's worth noticing that Nintendo has noticed what is taking Metroid sales down, which is it having too much adventure elements, which can be seen by Corruption made as a more linear game than the previous installments in the series.

How to get the sales up, would propably require the series to turn into more action, less adventure, but if it can perform to level near 3 million in sales, i don't think there's really any need to make any dramatic changes into the series. But who knows, maybe they want the sales up to 5M level (although, if the Metroid Prime collection is coming out, it, then again, shows that Nintendo is willing to push the series "as is", among luring the "hardcore" for the console).

60 Hz isnt a factor in the sales, your TV would have to be +20 years old to not accept 60 Hz signals.

Metroid is a very core franchise. Doesn't appeal to the mainstream so its hard for it to generate a lot of huge sales. One of the few shooters that are like that actually.

Screw that! I want more adventure, more exploration, but less back tracking. It still should have back tracking just not as much because levels > back-tracking.

@ksv: Not really. The 60Hz started to be more common only in widescreen TV:s (they should all support it), while in 4:3 screens it's pretty random what it puts out, in some TV:s you only see "not compatible with 60Hz mode", some gives you only flicker, some don't show you the colours (could be PAL/NTSC compatible TV) and stuff like that. Even some newer TV:s have trouble with PAL 60, since some (PAL/NTSC compatible) have autodetection for the input signal and they read PAL 60 as NTSC, when the colours often show as B&W.
Some 4:3 TV:s do support PAL 60, but never actually seen one.

funny....I thought the last game didn't have enough back tracking.  Part of what makes the back tracking work is that it is so easy to get lost in a Metroid game.  But while you are lost you are constantly finding items/ better routes to get places.  After all the back tracking you rtruly feel like you understand the world of Samus.

But I don't know how to make the series sell any better.  Multiplayer would certainly be a good addition...as would a leaderboard/rankings for different completion times/percentages.

I think more Zero Suit Samus would boost sales :P

@Super Etecoon: Echoes and Hunters had multiplayer. The multiplayer actually is one of the "action" elements.

@Liquidninja: Backtracking is one of the exploration elements, as Super Etecoon said.
But anyway, i too want more of the adventure elements, i just made the notion that more action focused games sell better in general than more adventure focused do.

i wouldnt want them to degrade the series into a typical shooter but it would surely help if they add a competent multiplayer, both on and offline.

so, keep the single player same atomospheric first person adventure but work on the multiplayer part of the game.

Yeah, Metroid is sort of stuck where it is. They would have to totally betray the spirit of the franchise if they wanted to appeal to the Halo crowd, and would equally betray it if they somehow tried to make it into a bridge title. They can't evolve the franchise in any sort of radical way, their only recourse is through creating an excellent game and advertising it accordingly