First and genuine reason is that its hard to plan with a group of people as its tough to fix the dates and schedule. And the second and the best one is in a solo trip you become truely independent and get to know the real you.
Its not about like or not. its about every trip either solo or group has their own charm. in solo u can find yourself more confidence because everything is managed by you and you are also enjoy a lot because you are going find yourself. in group you are going to enjoy each and every person taste so here you learn how to manage with other mood, choice etc. last but not leat, trip is for refershment so get a chance either solo or group just run ...