There still is I think (via Cranfield). You can also travel by train via Bletchley though connections are typically quite poor.
That's an even slower route which primarily serves the University at Cranfield (which I'd forgotten about). That's improved from less than hourly to twice an hour in term times, but takes around 90 minutes.
When the X5 started it replaced the X4 which took about 45 minutes between Bedford and Milton Keynes bus station via Bromham - I've checked an old timetable and seen it was only hourly around the peaks, with 2 hour gaps in the middle of the day.