Because you're not quoting your values. It has nothing to do with stripslashes or php in general, but basic html. Think about it. Given this:
<input value=Some Thing With Spaces name=field>
How's the HTML supposed to know where that ends? That's why you should use quotes.
<input value="Some Thing With Spaces" name="field">
Also, the type for the field should just be "text", not textfield. Unless you're really going for a textarea, which should be a textarea tag.