The initial set of programs was created around 1997 by Ignacy Misztal to

• help in teaching a course in mixed models
• simplify programming
• enable models not supported by earlier programs

Details on programming and computing algorithms are available in an Interbull paper by Ignacy Misztal Complex Models, More Data: Simpler Programming?

Features of Fortran 90 enabled a dramatic reduction in complexity compared to earlier versions of Fortran. Additional simplifications were achieved by putting less emphasis on low-level computational efficiency.

Over time, more and more programming was done by many people (see here), especially Shogo Tsuruta, Ignacio Aguilar, Yutaka Masuda, and lately, Matias Bermann.