du könntest is actually not past tense. The indicative past tense (Indikativ Präteritum) would be du konntest (without the Umlaut). The Konjunktiv II in past tense would be du hättest gekonnt (using the Indikativ Plusquamperfekt du hattest gekonnt as the base form and transforming the auxiliary hattest into its Konjunktiv II form hättest).
The Konjunktiv II Präsens, du könntest, is just formed bases on the Indikativ Präteritum. But still, this is present tense.
Konjunktiv II is used here as a means of extra politeness. In German, as in many languages, "softening" a statement and making it more indirect makes it more respectful, friendly and polite. Konjunktiv II is transforming the Indikativ into a mood of mere possibility, hence softening it and leaving / opening some space for exceptions.
The forms in the conjugation table you linked in the comment are correct, but the heading is at least misleading, if not bluntly wrong. They write "Präteritum", but this is actually wrong. I guess, the headings say where the form is derived from and they call it "Konjunktiv Präteritum", because it is formed on the base of the Präteritum form. (They also call the Konjunktiv II in the past tense "Plusquamperfekt", and it is formed on the base of the Plusquamperfekt form.) The name is misleading insofar as semantically this is still a present form, not a Präteritum form.