The answer is quite simple.....
Western saddles were designed so that people could spend all day, day after day in the saddle. The old cowboys would often spend 14 and 16 hours a day, every day for a month in the saddle. That is not something that you would want to attempt in an English saddle. The closest things to an English saddle that are used much on the trails are the Aussie saddles and the old time plantation saddles (though some of those are more western than others). Both the Aussie saddles and the plantation saddles though, have a considerably wider seat than the english, which enables the rider to sit longer with more comfort.
The horse too is more comfortable with your weight spread out over a larger area (provided of course the saddle fits properly). That too was a consideration when the western saddles were designed. An uncomfortable horse is a horse that tires quicker and responds less swiftly.
Though it is true there are uncomfortable western saddles, you will find very few if any serious trail riders that prefer English. This is especially true on long trail rides. For example, our typical ride is about 10 miles and occasionally we do 20 mile rides. I have found that most people who use english saddles on trail rides tend to ride for 3 to 5 miles at most. Not always true I am sure. But is most of the time. By contrast, there is a local trail ride hosted every year known as the 100 mile ride. It starts at Emminence Missouri and ends in Mountain Grove, Missouri. It takes five days to ride if you stay with the group. They camp each night at a different location after covering 20 miles that day. Now that may sound easy, but I assure you that after five days, you are going to be sore unless you are very used to it.
@gale hawk When you put up your western saddle, turn the stirrups a little farther than they would be turned while riding and put a broom handle through them if the stirrup leathers are leather. Store it this way and the stirrups will be turned already for you and it will not cause the problems.