One reason could be that it is easier to find developers that has got experience with Spring. That could be pretty important as well if we are talking multi-year-multi-developer projects where there is a risk that the members of the team will be replaced over time. There is probably more books you can buy and read about Spring.
I cannot se any obvious technical reasons for not choosing Play though, any library that is available to Spring is probably available as a standalone java library that can be used directly from a Play project.
Also if we are talking about coding in Scala I'd certainly recommend Play over Spring.