It is because JS uses double precision for the numbers, but if you want to perform operations with bits, the task is nontrivial, so a specific instruction to convert JS double into integer makes the thing easier.
This ARM link explains it very well: https://community.arm.com/processors/b/blog/posts/armv8-a-architecture-2016-additions
In order to add more information regarding fuz’s comment, the differences between FCVTZS and FJCVTZS (both of them convert floating point to int) are that in case of overflow, FJCVTZS value will be 0x80000000 instead of overflowing. Furthermore, FJCVTZS can generate an exception in order to indicate how the conversion was (i.e. inexact).
FJCVTZS : http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0801g/hko1477562192868.html
FCVTZS : http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/FCVTZS_float_int.html