Mums always know best ! Even when the wrong
The more you tried the more likely she is to keep saying no !
Don't talk about them for a week or two . then ask her that you really would like a pair and you wouldn't act stupid on them
Tell that there safe, more people hurt themself on bikes and that you'll always wear safety pads ( Which you should do anyway ! ).
If she still says no just ask nicely why she doesn't like them. Say that even police think they look like good fun safe