Wooden floors are important because they provide the best support for feet. Undoubtedly, wood surfaces are best for executing proper fumikomi ashi. Try it yourself: train in a concrete floor and then train in a wooden floor and you'll notice a slight change in foot pressure. This minor variation is critical for proper footwork and balance and it has to do with floor's hardness.
Repeated practice in concrete floors can result in kidney, leg, knee and feet pain and you may even stop kendo altogether because of it. Advice: if you cannot practice kendo in a wooden floor always remember to wear protective feet gear. Keep looking for other surfaces to practice besides concrete floors. If you can't find a good quality location try placing inexpensive martial arts mats or karate mats but don’t get accustomed to them. Wood floors are essential to kendo so keep searching for a place that has them.