Does it really matter? Its just another day for them, ultimately.
Most people forget despite intending to wish the other a good bday and others are not too bothered to remember anyway.
I could understand if you kept on reminding them, but they chose to ignore it- but is that really the case? Most people know a lot of people and birthdays come and go all the time, some people are better at remembering all these random dates than others.
Not let your happiness be contingent upon the actions of your friends, especially over something as small as this. Make a joke out of it the next time you see them, they'll feel like idiots.
p.s.
Happy Birthday- have a gd one