Dear Harryss!
For any society to change its style of clothes etc is governed by circumstances prevailing at that point of time. So is the issue of Topi vs Dastar.
Dastar is a protective helmet Topi is not. Sikhs were organised as a Martial order to fight the might of Jahangir. The process of was commenced by 5th Nanak and crystallized by 6th Nanak, who fought 4 wars against Jhangir's oppression of the Indian masses
Sikhs are a Martial order of Almighty and no Army can go by or approve individual fancies as far as standard uniform is concerned. If as on date Dastar is the item of Khalsa uniform, stressing on topi etce is not only illogical but dead against the very spirit of Khalsiat, the StiGurBani which ordains that Sikhs must blend with the environment without sinking into it. Hope this helps. Feel free to share further on any issue.