Doh! Because it's more expensive, in terms of unit cost (excluding R&D which has been paid for). It's not spend-to-save either, 'cos the mighty F-15E's operating costs are far in excess of those contractually guaranteed by Eurofighter GmbH.
If we need more FGA/OS aircraft quickly then step one is to run on the Jag Force by using the GI airframes from Cosford (these have on average 2,000 Flying hours 'on the clock' (of 7,500 hours cleared) and two have already been converted to GR3A standards, at a cost of c.£450k each).
The second stage is to buy Gripen.