It isn't hard to see why. 100% invested mandates and limiting funds to specific styles (asset, country, size, sector, etc) limit performance even further.
Imagine you run a mutual fund, required to be 100% invested in S&P 500 companies. You are also required to hold at least 30 companies in the portfolio at any given time.
Sorry -- but not too many people would still be positive for the year with those sorts of requirements.
It isn't always the managers -- it is the industry. Who better to know when to be in cash that a manager who specializes in that style? And yet we, as investors, require them to be 100% invested, because otherwise we don't want to pay management fees.
But I know a lot of people who would have preferred to pay the 1% management fee and only be down 1% for the year, rather than pay the 1% management fee and be down 40% for the year...
More importantly, however: you're, not your. The former is a contraction between 'you' and 'are'. The latter is a possessive form.