Before the Ark is opened on Shabbat, we don't just say "veyahi b'n'so'a ha'aron" as we would on a weekday. We begin with Ein Kamocha. The Artscroll siddur (I don't have the page number handy) explains that we do this on Shabbat because we have more time for davening than we do on a weekday.
If this is the case, then why don't we also do it at mincha on Shabbat?
Is there a different reason to explain the addition in Shacharit that would explain excluding it during mincha?