I'm very surprised that people haven't named the basics of capitalism: Supply and Demand! Many have stated reasons that are definately a factor, though. The fact is that in a niche market, supply and demand take on a new angle. Let's say that you can create any 4' Normal output flourescent bulb for 1 dollar each (manufacturing cost). To make the basic (warm colors ~3400K temp) bulb for home use you have a $1 cost, and because there are 2500 (made up figure) major manufacturers creating the same product for the same market, you have to compete with their price. Your supply costs are $1 and your demand isn't really all that high for (specifically) your product. If you raise your price because of whatever reason, people can get a comparable product elsewhere (usually 1 shelf over in the store)!
Now let's say you want to make a (cooler temp ~ 10,000K bulb, or even a plant bulb ~5000K to 6500K) aquarium specific bulb and spend the same $1 to manufacture it. Your supply COULD be (theoretically) as high as for any other bulb, but your demand is much lower (to general consumers) and this weeds out many manufacturers from the competition circle because they don't see it worth while (for their desires) to create these lower sales bulbs, even at higher prices (hey, their choice!) Now you take into account a niche market (not everybody does it.....hobbies are great examples) and you have a small group of people who will pay much more just to meet their desires or hobby needs. The fact that these $30 bulbs in the aquarium hobby still sell is my proof! In a smaller market their is a markup due to increased need to "break even" and have profit. Just consider that to break even you needed to sell a whirlygig (classic sales example here!) at $3.75 and for a small profit you charge $4. If across town someone else was selling theirs for $10 and they were selling PLENTY, would you raise your price or settle for what is a "fair" profit? You have to understand that in niche markets, sometimes items can sell at rates similar to non-niche items that sell so fast stores can't keep them stocked! I just had a flashback to the "tickle-me-Elmo" toys! Some products have been in such demand that people stood in line (in tents and sleeping bags) for 3 days or more just to get one. You better believe that if I had been selling that product, I would have charged 2x the cost for the first 2 weeks or so! Some people say "that's highway robbery!", but when you think about it, I'm not holding a gun and you are coming to me! The same with these niche market manufacturers. I guarantee that prices would drop if everyone understood the basics of light bulbs and would just shop a hardware store for their bulbs. I don't think they would drop quite as low as everyday house bulbs but I'd think not more than 2x the cost.
Anyways, this is getting long and I hope this helps!