Why is Tisha B'Av more strict then other fast days?

On Tisha B'Av in addition to not eating and drinking we have additional restrictions as well such as not wearing shoes, washing etc., why is that?

There is a fundamental machlokes regarding this.

The Gemara in Rosh Hashana (18b) quotes a pasuk in Zecharya which calls the 4 fast days both a צום and ששון ושמחה. The Gemara asks that the 2 contradict each other. The Gemara answers when there is shalom then these days are holidays (ששון ושמחה). When there are גזירות on the Jewish people, צום, when it is neither then רצו מתענים רצו אין מתענים. The Gemara then asks what about Tisha B'Av? The Gemara answers that Tisha B'Av is different because הוכפלו צרות and therefore if it is not a time of shalom you have to fast.

The Ramban explains the following based on the Gemara. Why do we fast at all today on the other 3 fast days, we are in that intermediate stage where רצו אין מתענים? He explains that the Jewish people accepted to fast on these days. Really, all 4 fast days really should be like Tisha B'Av with the 5 עינויים. However, since now there is no חיוב to fast, the RAmban says that the Jewish people did not accept a full fast but a partial fast and did not accept the other עינויים.

Others argue on the Ramban and say that really the only prohibition on all fast days is eating and drinking. Tisha B'Av is different because we add in halachos of Aveilus. The additional issurim on Tisha B'Av are issurim of Avelus.

There is a very big נפקא מינא between the 2. According to the Ramban during a time like the Holocaust there is a חיוב מדינה to observe the other 3 fast days like Tisha B'Av while according to the others there is not.