Because は (ha) in this instance is originally the Japanese topic marking particle. Whenever the Hiragana character は (ha) appears as the topic marking particle it is pronounced like わ (wa). ^^ But in regular words it is just pronounced 'ha'. こんにちは literally translated would say "as for today". The こん is like 'this'; the にち is 'day', and the は is marking 'this day' (today) as the topic, so therefore こんにちは. ^^

Miss Katt. It is so gooooooood answer. Great !!!