Why does everyone wear Rambam if they are Ashkenazi. What is the reason why many Rabbis pasken like Rambam over Tosafot?
First of all I am not sure why you believe “everyone” wears Rambam; according to our sales figures: 25-30% wear Rambam, 70-73% Raavad, and 1-2% Tosafot. (I think Raavad is the strongest seller partly due to the fact that we ship Raavad as our default).
Now though you are correct in your assumption that for the most part Ashkenazi poskim hold like Rashi/Tosafot in general, this does not mean that they never poskin like the Rambam. And in fact there are many who argue (e.g., R. Rabinovitch of Maale Adumim, R. Dov Lior of Hevron) that when there is no defacto psak (R. Dov Lior used the expression “ain messoret shel psika”) even Ashkenazim defer to the Rambam.
There are many logical arguments for and against each of the opinions (Rambam, Raavad, Tosafot), so much so that R. Schachter of YU states that Ashkenazim should go like Tosafot and Sefaradim like Rambam. That being said, many do find the reasoning of the Rambam more compelling (see R. Yehuda Rock’s article https://www.tekhelet.net/pdf/rak.pdf). Furthermore, as explained in R. Rapport’s article (https://www.tekhelet.net/pdf/ra02.pdf), since we are not 100% sure which is the right method, the Rambam’s is the most conservative (i.e., avoids possible violation of not
having enough white). And finally, one last explanation may be financial – don’t forget that Tosafot uses twice as much tekhelet and so the strings cost twice as much per set.
– Mois Navon.