Why can't i install mongodb 3.0 on Ubuntu 14.04 32bit?

There are no 32 bits packages in those repositories:

Check for yourself, amd64:

wget -O - \
  http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.1/multiverse/binary-amd64/Packages.gz \
  2>/dev/null | zless | grep ^Package
Package: mongodb-org-unstable
Package: mongodb-org-unstable-mongos
Package: mongodb-org-unstable-server
Package: mongodb-org-unstable-shell
Package: mongodb-org-unstable-tools

Whereas for i686, no packages are returned.

wget -O - \
  http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.1/multiverse/binary-i386/Packages.gz 2>/dev/null

Or if you prefer clicks, open a browser to that location:

xdg-open http://repo.mongodb.org/apt/ubuntu/dists/trusty/mongodb-org/3.1/multiverse/binary-i386/

Also, check this one.

Moreover, the vendor discourages the use of 32 bit binaries for production use, probably due to its limitations:

For production deployments, always run MongoDB on 64-bit systems.

based on mongdodb docs you can try to install the tarball installation. That is (taken from the docs):

1. download the binary files here or using curl -O https://fastdl.mongodb.org/linux/mongodb-linux-i686-3.0.6.tgz
2. extract the files tar -zxvf mongodb-linux-i686-3.0.6.tgz

3. copy extracted files

   mkdir -p mongodb
   cp -R -n mongodb-linux-i686-3.0.6/ mongodb
   

4. add it to PATH

   #example in ~/.bashrc
   export PATH=<mongodb-install-directory>/bin:$PATH
   

note as noted in mongodb using this method will have some limitation

"This 32-bit legacy distribution does not include SSL encryption and is limited to around 2GB of data. In general you should use the 64 bit builds."

but in the end it is up to you.