It is because it will be extremely expensive to do this. Plus there are a ton of issues to be worked out before this is possible.

Here is just a small list:

- What food will people eat on Mars?

- What space suits will they all use?

- Who will train all the astronauts to go there?

- Who will build the habitats?

- Who is building the equipment to make fuel on mars? How much will this cost?

This is far from a fully researched plan. Look, I really hope it happens and I'm seriously rooting for SpaceX, but there is much more to figure out and plan to make this a reality.