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If you want to know why, think beyond just desktop systems. There is Linux now on PDA's, tablets, phones and so on. Executing 99% boiler plate code over and over again doesn't make any sense.

Package maintainers already have written init scripts. They don't have to convert. If they want to take advantage of systemd native service files, they could write one as well but this is hardly going to take any time at all. Here is an actual example

NetworkManager.service file for systemd
----------------------------------------

[Unit]
Description=Network Manager
After=syslog.target

[Service]
Type=dbus
BusName=org.freedesktop.NetworkManager
ExecStart=/usr/sbin/NetworkManager --no-daemon

[Install]
WantedBy=network.target multi-user.target
Alias=dbus-org.freedesktop.NetworkManager.service

----

The equivalent sysvinit script is:
-----------------------------------

#!/bin/sh
#
# NetworkManager: NetworkManager daemon
#
# chkconfig: - 23 84
# description: This is a daemon for automatically switching network \
# connections to the best available connection.
#
# processname: NetworkManager
# pidfile: /var/run/NetworkManager/NetworkManager.pid
#
### BEGIN INIT INFO
# Provides: network_manager $network
# Required-Start: messagebus
# Required-Stop: messagebus
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: start and stop NetworkManager
# Description: NetworkManager is a tool for easily managing network connections
### END INIT INFO

prefix=/usr
exec_prefix=/usr
sbindir=/usr/sbin

NETWORKMANAGER_BIN=${sbindir}/NetworkManager

# Sanity checks.
[ -x $NETWORKMANAGER_BIN ] || exit 1

# Source function library.
. /etc/rc.d/init.d/functions

# Source network configuration
. /etc/sysconfig/network

# so we can rearrange this easily
processname=NetworkManager
servicename=NetworkManager
pidfile=/var/run/NetworkManager/NetworkManager.pid

RETVAL=0

start()
{
echo -n $"Setting network parameters... "
sysctl -e -p /etc/sysctl.conf >/dev/null 2>&1
success
echo

echo -n $"Starting NetworkManager daemon: "
daemon --pidfile $pidfile --check $servicename $processname --pid-file=$pidfile
RETVAL=$?
echo
if [ -n "${NETWORKWAIT}" ]; then
[ -z "${LINKDELAY}" ] && LINKDELAY=10
echo -n $"Waiting for network..."
nm-online -q --timeout=$LINKDELAY || nm-online -q -x --timeout=30
[ "$?" = "0" ] && success "network startup" || failure "network startup"
echo
[ -n "${NETWORKDELAY}" ] && /bin/sleep ${NETWORKDELAY}
fi
[ $RETVAL -eq 0 ] && touch /var/lock/subsys/$servicename
}

stop()
{
echo -n $"Stopping NetworkManager daemon: "
killproc -p $pidfile $servicename
RETVAL=$?
echo
if [ $RETVAL -eq 0 ]; then
rm -f /var/lock/subsys/$servicename
rm -f $pidfile
fi
}

# See how we were called.
case "$1" in
start)
start
;;
stop)
stop
;;
status)
status -p $pidfile $processname
RETVAL=$?
;;
restart)
stop
start
;;
condrestart)
if [ -f /var/lock/subsys/$servicename ]; then
stop
start
fi
;;
*)
echo $"Usage: $0 {start|stop|status|restart|condrestart}"
;;
esac
exit $RETVAL

----

The difference is enormous. systemd code has extensive support for sysvinit scripts so that it can act as a drop in replacement and it is perfectly fine if most of the system continues to use sysvinit scripts. systemd will support them forever essentially.