I found this paragraph on a recent decision by the CJEU (Case C-264/18):
24 According to the Court’s settled case-law, the purpose of coordinating, at European Union level, the procedures for the award of public contracts is to eliminate barriers to the freedom to provide services and goods and therefore protect the interests of traders established in a Member State who wish to offer goods or services to contracting authorities established in another Member State (see, to that effect, the judgment of 13 November 2007, Commission v Ireland, C‑507/03, EU:C:2007:676, paragraph 27 and the case-law cited).
I would dispute the merits of the procedural focus, but other than that this statement is spot on. Procedures are coordinated to reduce trade barriers, effectively meaning the use of Art. 114 as grounds for the Directive is perfectly correct.