What sort of markets breed multiple therapies? The ones with the largest number of potential (paying) patients, for one thing, which shouldn't surprise anyone. Fortunately, that also corresponds pretty well with the markets that could be better served with another drug. The success of the first compound in a new market shows that it can be done, and proves that there's money to be made, and that attracts more companies to the field. The path has been cleared, up to a point - but remember, all the following compounds have to go through the same degree of efficacy and safety testing as the first one did, although the later entries at least know, broadly, how to set up their clinical trials.
But entering an existing market is no guarantee of riches. There are no guarantees of riches. Look at Bristol-Meyers Squibb and their statin, Pravachor, which has been weighed in the balance against Lipitor and been found wanting (with BMS paying for the whole process). Now what? You can be sure that AstraZeneca is beavering away, trying to show that their big statin hope (Crestor) has even more of the effects that Lipitor has shown. It'll need to.
Or look at the erectile dysfunction field. Pfizer showed that it could be a moneymaker with Viagra, and don't believe for a minute that this was obvious beforehand. There were plenty of sceptics, doubtless some of them within Pfizer itself. But Viagra's success attracted a lot of interest around the industry, since many companies had a stable of potential PDE inhibitor leads. Now Bayer (partnered with Glaxo SmithKline) and Icos (partnered with Lilly) are in the field with Levitra and Cialis, respectively. A glance at your e-mail spam will have already familiarized you with both compounds, in case you've somehow missed the massive advertising campaigns.
The hope was the the new compounds, which differ from Viagra (and each other) in side effects, onset, and duration of action, would expand the market even more to people who had never tried Viagra. But as far as anyone can see, neither of them has been the as big a hit as the companies involved might have hoped for. It looks like the size of the market is still roughly the same, only now there are three compounds beating each other up inside the same box. The promotional costs for everyone involved are not trivial, and they would have been a lot easier to bear if a lot more patients had decided to try the therapies for the first time. It doesn't seem to have happened, at least not yet. Those are the breaks. We make money in this business, true, but if you think it comes easy, well, come on down with your leaf blower and scoop some up.