This question begins with two assumptions - assuming there is no issue of beged isha and assuming that pants are immodest (which is the position of many orthodox women).
Is there a reason why this would be permitted?
It was suggested that the issue of pants is dat yehudit, which would depend on what how women usually dress. Perhaps in this setting, modest women did not accept this upon themselves.
On the other hand, perhaps pants are inherently immodest, irrespective of what women do. If this is the case, perhaps wide pants would be different.