From the photos from the scene it would've been a lot easier to climb on the balcony around back and thru the doors or the kitchen window. Something RG was familiar with because he had been to the boys flat.
It maybe easiest to climb but it's not the safest or the fastest. It's also exposed and there's no quick escape route from the balcony.
The window Guede chose was safe and easy to break and very easy to climb and enter in seconds as the guy demonstrated on TV.