Mainly because there was a huge publicity campaign for breast cancer, and indeed it could be argued that it has disproportional amounts of funding. It also got a lot of publicity from the feminist movement.
Testicular cancer, however, is a poor example to use. Testicular cancer is already highly curable with existing treatments. Better examples would have been prostate cancer (which kills about as many men as breast cancer kills women) or lung cancer (which kills more than both of them put together).
Quick question and statistic for you: did you know that more men die of breast cancer than die of testicular cancer?
In the US an average of 450 men die from breast cancer each year; an average of 350 men die from testicular cancer.
So clearly sexism isn't the issue here
Breast cancer has a higher profile than other cancers, yes; and the reason is simple- sheer hard work.
Breast cancer awareness campaigns and Awareness Month started as a campaign by ordinary women, most of whom had breast cancer or had lost someone to it, to raise awareness so that people knew the symptoms, examined themselves regularly, attended their routine mammograms etc. It caught the public imagination and enthusiastic participation and hard work by women made it grow into something nationally, then internationally, recognised.
And then big business cashed in...
There is nothing to stop you or anyone who feels passionately about testicular cancer from starting such a campaign - they'd have to be prepared to work as hard and be as dedicated as those women though.
The Movember campaign is an example of someone doing just that. It's a sponsored moustache growing event throughout each November to raise awareness and money for men's health, especially men's cancers
The founder of the Movember movement noticed the success of breast cancer campaigns and copied it. He openly acknowledges his debt to (and admiration for) the hard work done by women around breast cancer. It inspired him to work similarly hard around the cancers he felt passionately about. And in 2011 Movember raised £22m in the 30 days of November in the UK alone.