Because, for beginners, practice is everything. You have to practice EVERY SINGLE DAY, even if it's just 15 minutes. Every day deliberate practice is the key to mastering anything, including guitar. And, as you already know, regular guitar isn't that easy to carry with you whereever you go.
I'd suggest you some of my favourite travel acoustics here but I've already done that on my blog post.
You can check it out here: http://acousticgo.com/top-5-travel-acoustic-guitars/
You will find other interesting articles for beginners there too ZZWinkZZ
Hope that helps you in any way.