Even though this question is out of the time frame....

[O'ETAIPOS]

Perhaps it was to expensive? anyway many tech were lost since the romans recruitted Barbarian units....

Tech were lost later, during wars of III-VI centuries that left a lot of regions in ruins, we should also remember that Great Desease in the end of II century destabilised all economy (and trus production) of Roman Empire.

[Philippus Flavius Homovallumus]

Far more important though were the wars themselves and the fact that the nessecitated Legions spending much longer in the field, being re-deployed and moved from bases they had occupied for more than two centuries. Rather like an SA80 LS is fine in barracks but on the march it is an unpleasent and potentially life threatening headache.