It's the merger that just refuses to happen.

On Monday, we learned from Bloomberg News that the French telecom operator Iliad was giving up on buying T-Mobile after the U.S. company rebuffed an offer valued at $36 a share. That's slightly higher than Iliad's first bid of $33 a share, but the increase evidently wasn't enough to get T-Mobile execs to hand over the keys.

What happens now? Analysts say Dish Network might have designs on T-Mobile, but nothing's been announced and likely won't be over the near term. Could there be other buyers out there? Maybe, but keep in mind that T-Mobile faces a big, underlying challenge — one that's effectively captured with two charts.

T-Mobile's made some promising moves by stealing customers away from other networks. With special promos like an offer to pay switchers' early termination fees, T-Mobile has begun adding users at a respectable clip. That reverses a long trend of folks abandoning the company for the likes of AT&T, Sprint and Verizon.

But even as the company has expanded its userbase, the cost of offering all those promos — not to mention the other initiatives that have become part of T-Mobile's "Uncarrier" campaign, such as abolishing contract pricing and adding free international data and texting — appear to be eating into the company's margins on a per-user basis.

What's basically happening at T-Mobile, analysts say, is unsustainable over the long run. It won't matter if T-Mobile has 20 million subscribers or 200 million if the company isn't making enough money.

Of course, part of the reason T-Mobile is making less is because it's investing heavily in its 4G LTE network, which now covers more than 300 cities and is still growing. The company is also fielding new wireless spectrum that it recently bought from Verizon, and it's also got its eyes on a bunch of spectrum real estate that the government will be auctioning off next year.

But all of those things are expected to pay off over the long term, and it's unclear what, if anything, can halt the slide in revenue that's happening in the here and now — which may be one reason T-Mobile is still waiting for its lucky break.