将最外层的RelativeLayout 替换成 LinearLayout 底部导航栏就消失，这是为什么？

最外层使用RelativeLayout 整个界面是正常的，但是换成LinearLayout ，底部导航栏就不显示，请问这是为什么？附代码如下：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical" >
    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="@android:color/white"
        android:orientation="vertical" >
        <include layout="@layout/main_title_bar" />
        <FrameLayout
            android:id="@+id/main_body"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="@android:color/white" />
    </LinearLayout>
    <LinearLayout
        android:id="@+id/main_bottom_bar"
        android:layout_width="match_parent"
        android:layout_height="55dp"
        android:layout_alignParentBottom="true"
        android:background="#F2F2F2"
        android:orientation="horizontal" >
        <RelativeLayout
            android:id="@+id/bottom_bar_course_btn"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1" >
            <TextView
                android:id="@+id/bottom_bar_text_course"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_alignParentBottom="true"
                android:layout_centerHorizontal="true"
                android:layout_marginBottom="3dp"
                android:gravity="center"
                android:singleLine="true"
                android:text="课程"
                android:textColor="#666666"
                android:textSize="14sp" />
            <ImageView
                android:id="@+id/bottom_bar_image_course"
                android:layout_width="27dp"
                android:layout_height="27dp"
                android:layout_above="@id/bottom_bar_text_course"
                android:layout_alignParentTop="true"
                android:layout_centerHorizontal="true"
                android:layout_marginTop="3dp"
                android:src="@drawable/main_course_icon" />
        </RelativeLayout>
        <RelativeLayout
            android:id="@+id/bottom_bar_exercises_btn"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1" >
            <TextView
                android:id="@+id/bottom_bar_text_exercises"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_alignParentBottom="true"
                android:layout_centerHorizontal="true"
                android:layout_marginBottom="3dp"
                android:gravity="center"
                android:singleLine="true"
                android:text="习题"
                android:textColor="#666666"
                android:textSize="14sp" />
            <ImageView
                android:id="@+id/bottom_bar_image_exercises"
                android:layout_width="27dp"
                android:layout_height="27dp"
                android:layout_above="@id/bottom_bar_text_exercises"
                android:layout_alignParentTop="true"
                android:layout_centerHorizontal="true"
                android:layout_marginTop="3dp"
                android:src="@drawable/main_exercises_icon" />
        </RelativeLayout>
        <RelativeLayout
            android:id="@+id/bottom_bar_myinfo_btn"
            android:layout_width="0dp"
            android:layout_height="fill_parent"
            android:layout_weight="1" >
            <TextView
                android:id="@+id/bottom_bar_text_myinfo"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_alignParentBottom="true"
                android:layout_centerHorizontal="true"
                android:layout_marginBottom="3dp"
                android:gravity="center"
                android:singleLine="true"
                android:text="我"
                android:textColor="#666666"
                android:textSize="14sp" />
            <ImageView
                android:id="@+id/bottom_bar_image_myinfo"
                android:layout_width="27dp"
                android:layout_height="27dp"
                android:layout_above="@id/bottom_bar_text_myinfo"
                android:layout_alignParentTop="true"
                android:layout_centerHorizontal="true"
                android:layout_marginTop="3dp"
                android:src="@drawable/main_my_icon" />
        </RelativeLayout>
    </LinearLayout>
</RelativeLayout>