[QUOTE][b]Yet today, most christians would say we should observe the ten commandments; few would say we should also observe the six hundred and something other instructions on how to live. Why?[/QUOTE]
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That was explained to me as being because, if the Jews couldn't hold themselves to all their laws, Gentiles shouldn't be asked to either.
Christianity is a more gregarious faith than Judaism and it is open to anyone (or really liberal Judaism for everyone + Messiah). Judaism, traditionally, is one based on family lineages and is not as gregarious. Sadly, this has caused them much hardship over the years.
Why do Orthodox Jews still keep to their strict laws? Well remember, they aren't Christians and so they don't believe in the Christian covenant with God established through the death of Christ.