Late to this discussion, and I predict my views won't be popular based on what I've seen thus far, but here goes.
The Imperial Army is standardized at TL12. which is "Average lmperial," and reflects the median of Imperial worlds. As such, the gear is easier to come by, less expensive, easier to maintain, and more reliable. Worlds have TLs for a reason and one of them is that they describe what is locally maintainable. From the CRB: Tech Level "measures the average technology presence on the planet and gives an idea of local production and repair capability." The army does not have a dedicated fleet of ships to bring in a constant flow of replacement parts and materiel to keep up their gear. They depend on commercial shipping and the navy to bring in higher tech gear, but that doesn't mean it's standard issue. It means it's available for elite forces and special circumstances.
A microcosm of this may be seen in mercenary units, which might have access to higher-tech gear, but often use locally available tech for longer deployments. If you're deployed to a TL8-9 world for a six-month security ticket, the logistics of maintaining your gear is made much more possible when you lean on local capabilities.
While the Imperial Navy is largely (but not entirely) TL15, the Imperial Army has considerably more personnel than the navy and materiel is expensive and hard to keep up. The army is spread across 11,000 worlds whose governments and legal apparatuses are largely independent of the Imperium. As long as they don't break the few (mostly vague) Imperial laws, they are left to their own devices. The presence of an all-powerful, maximum-tech army on all those worlds is directly opposed to that notion and impedes with the narrative.
Where necessary — conflict zones, the border, etc. — there are exceptions. If you expect to be in a conflict (i.e., you're in a "hot zone"), armies can be equipped to the maximum Tech Level possible. "Technical Maximum Imperial" is TL15. Even army troops on TL12 worlds in the interior may be trained on TL15 gear over the course of their service, so they are prepared to use superior equipment when called upon to do so, which is rare for 99% of the army.