as moab man pointed out, wedding photography needs specific skills that photography areas dont use. sometimes its not even about photography. a lot of humane skills are involved and a lot of psychology as well.
wedding photography is extremely stressful at times and immense pressure to get shots. people stand in front of you and you need to arrange them, and guide them and make them look the best without any hesitation or fumbling with gear. youre extremely constrained with times and people are late many times where you have to keep your cool and still get stellar images under a very short time.
I did a wedding recently when the groom came late to pickup the bride and then we usually go do an outdoor studio shoot of the BG. usually I need an hour because the bride needs her time more to adjust her dress and walking from an area to an area, she needs to lift her dress and it just takes time to pose people for pictures. you cannot rush it. people are emotional that day and you need to do it gently while inside you want to scream "HURRY THE FEK UP I HAVE NO TIME! IM LATE TO SHOOT THE FAMILY FORMALS!" and you need to stay cool. dont even get me started on spolied brides who think (they arent though) their princesses and they just complain and dont cooperate. I said wow theyre going to have some shetty images. 20 minutes to shoot. I need time to create nice images. loe and behold, I transfer images in the evening to HDD and damn some nice images I was able to produce. some that are going to my website. and thats because of experience and knowing how to work under stress.
wedding photography IMO is one of the most demanding and time constrained photography out there. and a good wedding photographer is good at many other photography areas as well. we shoot under low light, we shoot with fashion photography in mind, we shoot portraits, we know flash and how to use it to its best. we shoot architecture we shoot like sports photographers. we are urban photogs, we know how to shoot still life, we shoot in the heat and cold and are on our feet for 12-15 hours. we have peaks and lows throughout the day and you go home fatigued. it takes me a full day to recoup from a wedding.