Running migrations from .install file

I am trying to run migrations from a module's install file.
When installing the module, imports do not appear to be processed and no content is created.
When copy/pasting the below code in Devel's Execute PHP block it works, imports are processed and content is created.

migrate_get_module_apis(TRUE);
$migrations = array(
  'Smallprint',
  'Page',
  'Service',
  'Link',
  'Testimonial',
  'Blog'
);
foreach ($migrations as $machine_name) {
  $migration = Migration::getInstance($machine_name);
  $migration->processImport();
}

Any ideas why this does not work in an install file, whilst it works using Devel's Execute PHP block?

To reproduce, create module:

mymigratetest.info

 name = My Migrate Test
 description = Imports Migrate Example content.
 core = 7.x
 dependencies[] =migrate_example

mymigratetest.module
<?php
mymigratetest.install

<?php
function mymigratetest_install() {
  $migrations = array(
    'BeerTerm',
    'BeerUser',
    'BeerNode',
    'BeerComment'
  );
  foreach ($migrations as $machine_name) {
    $migration = Migration::getInstance($machine_name);
    $migration->processImport();
  }
}

Installation of the module will not create content, whereas installing migrate_example and pasting the below code in Devel's Execute PHP block will.

    $migrations = array(
    'BeerTerm',
    'BeerUser',
    'BeerNode',
    'BeerComment'
  );
  foreach ($migrations as $machine_name) {
    $migration = Migration::getInstance($machine_name);
    $migration->processImport();
  }

Any ideas why this does not work in an install file, whilst it works using Devel's Execute PHP block?