That worked out wonderfully for them the last time they tried it.
Actually, in the last round of municipal elections, in most cities Degel ran separately from Agudah, and did amazingly well.
To summarize a very complicated situation in a few sentences: There are long standing agreements on division of power and representation between the Degel and Aguda. This is both nationally, and in every city with Charedi representation on the town council. Aguda, internally, also has representation agreements between the different chassiduses that make up the Aguda. Most of the Degel/Aguda agreements date back to the 1980s, while much of the internal Aguda agreements date to the 1960-70s.
There is/was dissatisfaction with both these deals, as they are based on the demographic makeup of the Chareidi world at the time the deals were negotiated, which some of the parties feel has changed since then. Obviously the sectors that feel that they will lose out if the deals are renegotiated, resist redoing the deals.
There also are/were claims that some of the parties to the deals are not keeping their side of the bargain, and allegedly cutting there own separate deals with other parties to benefit their sector, at the expense of the common agreed platform and priorities. (Gur and Barkat in Yerusholayim is one of the claims)
Due to this, and other considerations, in many cities Degel ran separately from Aguda. In many places, the degel share of the vote far outstripped the proportions that the deals were based on and in most cities the Degel supported candidate for mayor won over the Aguda supported candidate. (Example: in JLM, degel received 9 vs Agudas 5, when the previous agreement barely gave them parity; Leon, supported by Degel won)