This is really quite simple, it's because it was the first. People have been so ingrained to believe that if you are 1st, you are number 1 and the best. Most people also believe that bitcoin will have the most value at the end of a 20 year run which adds even more to the value. I believe that all of the forks are only adding more value because everyone is to freaked out to invest tons of sums because if it doesn't work out, they lose all their money in the new forked coin. I think the whole world has heard of bitcoin, but not everyone believes in it yet.
I think another big time reason is because their has already been to much value pumped into bitcoin, for example, people have bought houses, land, cars with bitcoin. My last point is because you can make a lot of money on bitcoin in the short term and the long term. The short term because all of the forks makes it fluctuate, long term because I can see it being used in a 100 years.