It's primarily due to cost. These particular crystals are dirt cheap due to the watch industry. This answer provides more detail, here's an excerpt:

There are 1.2 billion watches sold each year. The majority of them are inexpensive digital watches, requiring a small, 32kHz crystal. ...

As a result, these crystals are extraordinarily inexpensive... [Other crystals] cost 10 to 100 times more in quantity than these inexpensive watch crystals.

Further, these crystals are particularly well optimized for low power. Real time clocks are expected to run such an oscillator for 10 years on a CR2032 type cell. To get low frequency, low power, small crystals in other frequencies, you're looking at a substantial increase in cost.

In low volumes these crystals are still less expensive than even the normal or high power the 25kHz or 56kHz crystals, but cost the difference isn't large until you get into high volume manufacturing.

Choose what you need, but if you are going to produce a high volume product and can adjust your design to work with a 32kHz crystal, then there's a substantial financial incentive to do so.

It's primarily due to cost. These particular crystals are dirt cheap due to the watch industry. This answer provides more detail, here's an excerpt:

There are 1.2 billion watches sold each year. The majority of them are inexpensive digital watches, requiring a small, 32kHz crystal. ...

As a result, these crystals are extraordinarily inexpensive... [Other crystals] cost 10 to 100 times more in quantity than these inexpensive watch crystals.

Further, these crystals are particularly well optimized for low power. Real time clocks are expected to run such an oscillator for 10 years on a CR2032 type cell. To get low frequency, low power, small crystals in other frequencies, you're looking at a substantial increase in cost.

In low volumes these crystals are still less expensive than even the normal or high power the 25kHz or 56kHz crystals, but cost the difference isn't large until you get into high volume manufacturing.

Choose what you need, but if you are going to produce a high volume product and can adjust your design to work with a 32kHz crystal, then there's a substantial financial incentive to do so.