Out of interest, I've just loaded a Juicebox-Pro v1.4.1 gallery into an iframe and the Expand Button seems to be disabled in this version (just like in v1.5.0).
The Expand Button is disabled by design when the gallery is loaded into an iframe to avoid a lot of potential problems.
There are several things to consider such as the values of useFullscreenExpand and expandInNewPage and the browsers and devices being used to display the gallery. (Please see the Expand Gallery Behavior for some more information. Expanding a gallery can be quite complicated!)
Also, not all users will be using up-to-date browsers which support the Fullscreen API (perhaps in corporate environments).
Juicebox supports IE 7 and later so we need to be sure that our user base will not encounter any unexpected or unusual behavior in older browsers. IE 7 is certainly a bit long in the tooth now but the Fullscreen API was first supported by IE in version 11 and there seems to be more than a few people still using IE 8, IE 9 and IE 10 (over 10% of the total browser market share).
I would normally recommend that you use the current version of Juicebox-Pro but if you find that Juicebox-Pro v1.4.1 works for you and you are not affected by any bugs which have since been fixed in subsequent versions, then you are certainly free to continue using v1.4.1.