> why not develop the tech to mine astroids and then use the resources for building up human presence on Mars
Because we have all the resources we need on Earth to build a base on Mars, and if we make good use of ISRU ("live off the land") on Mars, we can get a lot of what we need right there. See for example synthesising methalox from Martian resources; marscrete; making glass and other chemical processes... Zubrin's "Mars Direct" book goes into many details here.
Building a reusable booster capable of launching vehicles that can do the Mars transfer is much more economical than building intermediate stages like orbital gateways or asteroid mines.
That's not to say mining asteroids is a bad idea though -- there are other companies looking at this too!
Because we have all the resources we need on Earth to build a base on Mars, and if we make good use of ISRU ("live off the land") on Mars, we can get a lot of what we need right there. See for example synthesising methalox from Martian resources; marscrete; making glass and other chemical processes... Zubrin's "Mars Direct" book goes into many details here.
Building a reusable booster capable of launching vehicles that can do the Mars transfer is much more economical than building intermediate stages like orbital gateways or asteroid mines.
That's not to say mining asteroids is a bad idea though -- there are other companies looking at this too!