So that they don't have to hang onto two separate versions of the same track, unless they plan to offer each version as a different layout. The team that works on the Gran Turismo franchise, if I recall correctly, is split between different versions as needed. Having multiple versions of the same track ultimately will lengthen development time, with the exception of reusing the GT4 version of both tracks mentioned in Mobile.
It just seems to me like a design choice made with the short term, rather than the long term, in mind. I say this because tracks that have seen redesigns in the past scrapped the older versions, rather than offering older versions as a separate layout. This would mean that Mobile is a fourth generation Gran Turismo game, whereas GT5 is, naturally, a fifth generation game (if you know what I mean), not that this distinction is a bad thing by any means. I simply find it odd that, with both games within a relatively short period of time from one another, they do not share the same track layouts for some of the iconic tracks.