This question came down the pipeline from the SEED overlords:
Why don't they make a birth control pill for men?
The short answer is that they do make various methods of contraception for men, but most of the more effective ones are surgical rather than pharmacological. Also, given the early difficulties in making a pill for men similar to the birth control pill for women, most of the pharmacological forms of male contraception are still in clinical trials to determine that they are safe and effective.
The history of a male birth control is filled with many false starts. For example, the Chinese noticed in the 1920s that an extract from cottonseed oil called gossypol was effective at reducing fertility in men. However, clinical trials in China during the 1970s revealed that gossypol caused permanent infertility in about 10% of men, so those trials were discontinued.
More modern pills use a strategy whereby the hormones necessary for spermatogenesis are blocked with specific inhibitors (GnRH and LH/FSH) and -- because these hormones also trigger testosterone production -- testosterone is replaced. This can take the form of just androgen administration, taking a Gonadotropin releasing hormone (GnRH) antagonist + androgens, or taking a progestin + androgens. These pills are effective and are going through clinical trials in other countries. However, they have three problems. Like the female birth control pill, they are not without physiological and behavioral side effects. (This is not in any way to suggest that female birth control doesn't have side effects. They do, and you can argue back and forth which side-effects are worse. Most of the side effects for males are similar to those for androgen use, i.e. increases in muscle mass, acne, aggressive behavior, etc.) Also, it often takes several months for the effective reduction of fertility. Finally, the long-term side-effects of androgen administration at these doses are not known.
Because hormonal contraception in men has proven complicated, barrier methods are being more thoroughly investigated. The most common of these is vasectomy -- which is effective with limited side-effects but can be permanent (vasectomy is only reversible in 50-70% of cases). Other forms of non-permanent vasectomy are being investigated, such as occluding the vas deferens with styrene beads -- also known as RISUG (Reversible Inhibition of Sperm Under Guidance). The vas deferens is a tube that the sperm have to go through before entering the uretha; it is tied off in a vasectomy. The styrene beads bead serve to block the vas deferens, and they are coated with a chemical that makes the sperm that make it through defective. However, the reversibility of this technology has not been tested in humans, and there are concerns about toxicology for both the beads themselves and the means of removing them. (It usually requires dissolving them in a chemical called dimethylsulfoxide [DMSO] which may be toxic.)
As a more general answer to the question, there are clearly both technical and sociological reasons why there aren't more widely used forms of male contraception. The technical reasons -- it seems to me -- tend to be on the difficulty in getting quick, effective, and reversible hormonal contraception to work in men. The sociological issues are more complicated and probably more meaningful. For many years -- and probably still today -- contraception is seen as the woman's responsibility, and that is clearly unfair. Hopefully that is changing.
For further reading about male contraception, I recommend this review. Also, malecontraceptives.org seems to have good information on options presented for lay people. I haven't read their whole site, but what I have seems reasonably accurate.
Men can get pregnant?
For many years -- and probably still today -- contraception is seen as the woman's responsibility
Why is it that I have never experienced this? No-one I have ever known or interacted with has ever displayed this attitude (that I know of). In my environment the idea has always been that men had better wear condoms or face dire consequences.
I suppose that, as Wodehouse said, half the world doesn't know how the other three quarters live.
Valhar2000 - I think that the two halves are singles and people in monogamous relationships.
IMHO couples tend to replace condoms (male responsibility) with the pill or IUD (woman's responsibility). This is not really due to the inherently patriarchal nature of relationships but because there is much less need to prevent STDs (which condoms are good for) and a greater risk of pregnancy-all that regular sex-which condoms aren't so good at preventing (or at least not as good as the pill or IUD).
In short, after a couple of burst condoms, the couple will often decide to find a more reliable means of contraception.